Confused about order of operations...

[Delmuir]

So I was hit with a staggering amount of bleeding damage (155 per turn in the old forest so... yeah, early for that kind of damage, although it was Insane) and it was going to kill me... but all I had available was a heal, which should have been enough to keep me alive. I cast it... and I died. The log never showed me casting it and nothing should have prevented it (and the enemy was already dead by then).

What happened? When do I suffer the damage versus when does the heal happen? Apparently, the damage came first but how do I figure that out in a situation like that?