"Behold, we die, we perish, we are all perishing! Everyone who approaches the sanctuary of God dies; have we stopped dying?" (17:27-28)
This complaint seems trivial. If the Mishkan is killing people who approach it, then just stay away from it. It's that simple! So why do the people bother complaining, why does the Torah mention it, and why does it (as we will see) receive a very long response?
Rewind to the beginning of Sefer Bamidbar. Excluding several "mitzvot for the generations", the first 10 chapters of Bamidbar contain the following stories:
1) Census of 12 tribes
2) Census of Levi and its role in the Mishkan
3) Gifts by princes at Mishkan inauguration
4) Aharon's role in lighting the menorah
5) Levites consecrated to serve in Mishkan
6) Pesach sacrifice performed
7) A Divine cloud covers the Mishkan, dictating when they travel or wait
8) The journey begins
9) Moshe asks Hovav to go with them; they travel, led by the Ark
10) What Moshe says when the ark stops/starts
These can be more or less divided into 3 sections:
1) Census in preparation for departure
2) Initiation of Mishkan
3) Journey, led by the Mishkan.
It is clear that at every stage, the Israelites are expected to follow the lead of the Mishkan in their travels. Whether they go or stop is determined by the presence of a cloud over the Mishkan. Which direction they go seems to be determined by the progress of the Ark. Physically the Mishkan is in the center of the camp. The departure from Sinai is delayed until the initiation of the Mishkan is completed.
It therefore seems that the Israelites have a very good reason for their complaint about dying whenever they approach the Mishkan. They have been trained to look towards the Mishkan as a source for guidance and an indicator of protection. But the death occasioned by Korach's rebellion associates the Mishkan with terror and punishment. How can the Jewish people be guided by something so apparently hostile to it?
As is usual in Sefer Bamidbar, these stories are followed by laws which relate to the stories. In this case, the ensuing laws include chapters 18 and 19. (The next story, and thus the next textual unit, begins with chapter 20.) Chapters 18 and 19, respectively, relate to the two stories in parshat Korach - 1) the long, multifaceted rebellion began by Korach which continues even after his death, and 2) the two-line complaint discussed above.
1) Korach's rebellion challenged the authority of Moshe and Aharon the priest. Chapter 18 reacts to this story by detailing the status and privileges of Kohanim and Leviim and the relation between the two groups.
2) The complaint indicates that the Mishkan had acquired a strongly negative association - with death and punishment. This complicated its role as the focus and guide of the traveling Jewish people. Chapter 19 - parah adumah - is designed to oppose this association. Through parah adumah, we learn to view the Mishkan not as a source of death, but as a source of purification from death. The negative association is replaced with a corresponding positive association, and the Mishkan can once again play its central role among the Jewish people.