Hm. Remember how memory charms, while removing the memories, don't necessarily remove the emotions of the erased moments?

Draco at the start of the chapter:

The feeling of emptiness that filled him up was so profound that it left no room even for pretended courtesy.

Everyone was dead.

Draco after obliviation:

The feeling of emptiness that filled him up was so profound that it left no room even for lies.

Everyone was dead.

Everyone was dead, and it had all been futile from the beginning.