Nobody knows (until after it dies) how many HP a monster has for me. If someone used a really cool power and the monster is down to at-wills, then it dies at 1hp. If it still has something cool left to do, then it lives at 1 hp.
On the other hand, our Rogue (Shadow Assassin) has the most incredible propensity for kill-stealing. That may be because she is very mobile, and has good ranged and melee attacks, or because monsters take damage when they miss her, or because she does more damage to bloodied targets.
In any case, in most fights, the little female halfling kills most of the monsters.
The last fight (DQE), she killed both the Death Hag, and the Goristro, without taking any damage during the entire fight (then she got freezerburn exploring the Hag's larder)!
That is kind of balanced by her ability to consistently crit on minions, and no one else. One fight, she killed 8 of 10 minions, and (IIRC) killed about 5 of those with crits.