The well-known story of Kamtza and Bar Kamtza (Gittin 55b-56a) goes as follows:
R. Johanan said: What is illustrative of the verse, Happy is the man that feareth alway, but he that hardeneth his heart shall fall into mischief? The destruction of Jerusalem came through a Kamza and a Bar Kamza; the destruction of Tur Malka came through a cock and a hen; the destruction of Bethar came through the shaft of a leather. The destruction of Jerusalem came through a Kamza and a Bar Kamza in this way. A certain man had a friend Kamza and an enemy Bar Kamza. He once made a party and said to his servant, Go and bring Kamza. The man went and brought Bar Kamza. When the man [who gave the party] found him there he said, See, you tell tales about me; what are you doing here? Get out. Said the other: Since I am here, let me stay, and I will pay you for whatever I eat and drink. He said, I won't. Then let me give you half the cost of the party. No, said the other. Then let me pay for the whole party. He still said, No, and he took him by the hand and put him out. Said the other, Since the Rabbis were sitting there and did not stop him, this shows that they agreed with him. I will go and inform against then, to the Government. He went and said to the Emperor, The Jews are rebelling against you. He said, How can I tell? He said to him: Send them an offering and see whether they will offer it [on the altar]. So he sent with him a fine calf. While on the way he made a blemish on its upper lip, or as some say on the white of its eye, in a place where we [Jews] count it a blemish but they do not. The Rabbis were inclined to offer it in order not to offend the Government. Said R. Zechariah b. Abkulas to them: People will say that blemished animals are offered on the altar. They then proposed to kill Bar Kamza so that he should not go and inform against them, but R. Zechariah b. Abkulas said to them, Is one who makes a blemish on consecrated animals to be put to death? R. Johanan thereupon remarked: Through the "anvatanut" of R. Zechariah b. Abkulas our House has been destroyed, our Temple burnt and we ourselves exiled from our land.
The beginning of the story (the meal) and end (the sacrifice) flow logically. But the segue between them is hard to understand. How is it possible that a random meal guest is able to show up at the king's door, and be taken seriously when he says the entire nation is rebelling?
R' Medan suggests a new approach to this story, beginning with a new interpretation of the word "anvatanut", usually translated as something like "humility".
Rashi on Brachot 9a explains that the "vatikin" who prayed early each morning were:
People with "anava", who loved mitzvot
If the two halves of Rashi's statement are related, than "anava" would seem to mean punctuality or enthusiasm for doing mitzvot in the best possible manner, not humility. [My addition: see Demai 6:7 where "tzanua" has the connotation of "machmir" - apparently, "frummies" who were known back then for cultivating modest character traits would also follow "machmir" rulings.]
We see a similar idea in the Tosefta (Shabbat 16:7) which discusses how to dispose of food waste after a Shabbat meal:
Beit Hillel says: One may pick up the bones and peels from the table. Beit Shammai says: One must remove the entire plate/tray and shake off its [muktzah] contents. Zecharia ben Avkilus did not practice like Beit Shammai or like Beit Hillel, but rather would take [an item of food to eat] and throw [its waste] behind the couch [so as not to have to remove muktzah items by any method later on]. R' Yosa said: R' Zecharia ben Avkilus' "anvatanut" is what burnt the Temple.
Here, R' Zecharia ben Avkilus and his "anvatanut" are characterized as extreme, unwarranted strictness with halacha.
Let us now look at Midrash Eicha Rabbati (on verse 4:2), which tells the Kamtza story again, and suggests another flaw of R' Zecharia ben Avkilus.
Once a man in Jerusalem made a meal. He said to his messenger: Bring me Kamtza my friend. He went and brought Kamtza his enemy. He entered and sat among the guests. [The host] entered and found him between the guests. He said to him "You are my enemy and you sit in my house, why? Get out of my house!" ... He took [Kamtza the enemy] in his hand and expelled him. R' Zecharia ben Avkulus was there and was capable of protesting but did not protest. He went out and thought "Since the aged rabbis were sitting there and did not object, they agreed with it." ... this is why it is said that the Temple was destroyed over Kamtza and Bar Kamtza. R' Yosef [=R' Yosa in the Tosefta] said: R' Zecharia ben Avkilus' "anvatanut" is what burnt the Temple.
This is basically the same story, told slightly differently. There is one addition of interest to us: the same R' Zecharia who refused to offer the sacrifice was responsible for precipitating the crisis by not protesting at the meal! But how can "anvatanut" refer both to this insensitivity and to the later halachic strictness?
Before answering this, let us look at a different paragraph in the same section of midrash.
Another interpretation: "The precious sons of Zion." ... What was their preciousness? One of them would not enter a meal unless he knew who was dining with him, who poured the wine, and who served the food."
This is a strange idea, but we find the same idea in the gemara (Sanhedrin 23a):
"Thus did those of clean mind in Jerusalem do: they would not sign a legal document unless they knew who was signing with them, they would not sit in judgment unless they knew who was sitting with them, and they would not go into a meal unless they knew who was eating with them."
It appears, from these sources, that was customary that guests would demand to know the entire guest list before coming! This shines an entirely different light on the Kamtza/Bar Kamtza story. People were very careful to never invite anyone who was not on an approved list. If a Bar Kamtza showed up uninvited, it's not at all surprising that he would be kicked out!
Why would people insist on knowing the name of every single person who attended a meal? Would you refuse to attend, say, a wedding, without first consulting the entire list of guests and waiters to see that every one of them was on your pre-approved list?
To answer this, let us remember that we are talking about the period immediately preceding the rebellion and destruction. Clearly, the political intrigues that would lead to the rebellion were already present. There were many spies around, and much suspicion and persecution. When wine goes in secrets come out, and for plotters, knowing every guest on the invite list was necessary for security. Also, it is quite reasonable to think that Bar Kamtza was just the sort of person who would be unwelcome in a feast of plotting rebels. Someone willing to betray his nation to the Romans after a personal insult at a meal was someone who probably not too loyal to begin with! As R' Yochanan said at the beginning of the story, "Happy is he who always fears" indeed!
The midrash fingers R' Zechariah as the rabbi most responsible for the expulsion of Bar Kamtza. If Bar Kamtza was a non-nationalist (willing to go straight to the Romans to avenge his insult) and his haters were nationalists, then perhaps R' Zechariah himself was a prominent nationalist. Let us now see what non-rabbinic sources may have to say about R' Zechariah.
Josephus (Wars of the Jews 4:4:1) writes the following about the zealots:
These leaders were Eleazar, the son of Simon, who seemed the most plausible man of them all, both in considering what was fit to be done, and in the execution of what he had determined upon, and Zacharias, the son of Amphikulus...
This name is very similar to R' Zecharia ben Avkulus, and some historians have asserted that they are the same person.
If so, R' Zecharia was not simply a modest, fearful man concerned primarily about the details of halacha. He was a polarizing zealot leader! No wonder he did not object to kicking out the unintended guest, who might have been a spy against the zealots. And no wonder he was not willing to compromise anything whatsover in the Temple service, even under extreme pressure.
Now, let us look about what Josephus says about how the revolt began (Wars 2:17:2).
At the same time Eleazar, the son of Ananias the high priest, a very bold youth, who was at that time governor of the temple, persuaded those that officiated in the Divine service to receive no gift or sacrifice for any foreigner. And this was the true beginning of our war with the Romans; for they rejected the sacrifice of Caesar on this account; and when many of the high priests and principal men besought them not to omit the sacrifice, which it was customary for them to offer for their princes, they would not be prevailed upon.
This account is very similar to that in the Kamtza story! In both, the Romans send an animal to the Temple to be sacrificed. In both, the Jewish leaders refuse to sacrifice it after extended arguments. In both, the Romans are provoked by this refusal and war ensues.
Of course, there are some differences. History writing in the ancient world was different from today. Nobody was looking over the historian's shoulder to see that everything was properly sourced and accurate. Rather, history was a form of what might be called "history art" where the details could be filled in as necessary. One famous example of this is the speech Josephus records as being given on Masada before the collective suicide. How could Josephus possibly know the text of this speech? If someone could bring Josephus back to life and ask him this, he would likely say - with justice - "I grew up with Elazar, I knew him well enough to know that this is what he would have said in that situation." Josephus did not know the exact words, but he was confident in his guess at the message. Similarly, Josephus filled in the gaps in the Jerusalem story as best he could. The essential story is the same as in the gemara (which testifies to the basic accuracy of both). But being in the Roman camp outside Jerusalem, and not knowing the details about Bar Kamtza, Josephus filled in other details about how the sacrifice came to be rejected.
In the gemara, the rabbis who R' Zechariah debated sound reasonable, while Bar Kamtza sounds like a spiteful traitor. But in Josephus' telling, the Jews who incited the sacrifice simply wanted to get rid of the zealots, not to destroy the Temple and the whole nation! The concrete facts are almost the same as in the gemara. But the tone and moral judgments are completely different. This reminds us of current events, where Haaretz and a right-wing paper might report almost the same events, but the presentation and conclusions to be drawn are near opposites.
The Torah tells us "You shall not bear tales ('rechil') among your people, you shall not stand by the blood of your fellow." (Vayikra 19:16) The two halves of this verse are likely connected. The tales in question may not be simple gossip about who got a bad haircut or woke up late for minyan, but the kind of betrayal to hostile authorities that can result in death [what was later known as "mesirah"]. And the word "rechil" may be linguistically related to "meragel", meaning to spy. Some sources say that "rechilut" or "lashon hara" was responsible for the destruction of the Temple (Yirmiyahu 9:3, Brachot 32b). If so, the kind of tale-bearing that is meant is the kind where people are being betrayed to the authorities.
We have now attributed clear political motivations to each side in the Kamtza story. Thus, it appears that the story is not "simply" an anecdote of baseless hatred. But while it is legitimate to have differences of political opinion, it is not legitimate to have so much mutual back-stabbing that members of your nation cannot even eat at a meal together. That degree of hatred leads to destruction and exile. Kamtza (who was welcome at the meal) and Bar Kamtza (who was unwelcome) represent the two political camps which showed so much hatred towards each other, and therefore the story is named after both of them.