Negative Damage - Noob question

[Bananadine]

Here's a thing I've seen toooo many times: Demilich's minions don't quite manage to kill the earth elemental trying to assassinate their master; the demilich goes off script; he casts Disintegrate a few times, succeeding once and "killing" one form of the elemental; the smaller elemental crushes him.

In that case a spell that's supposed to kill something doesn't kill it at all due to the form change... but the damage is approximately (or maybe exactly--is that how Disintegrate works?) the target's HP count, and not 999, so it only seems a little weird that the target doesn't die, and not highly weird as with the Soul Slay example.

Dang now I want to test this stuff (but it's too late for such shenanigans tonight)