Q u o t e: It should be one of those situation IMO :
- The 1 hit KO require your character to be very low on health.
- If his standard attack get your health to 0, it chain into the grab that execute you.
I was going to post this exactly.
It's clear that enemies having the ability to one-shot you, assuming you have plenty of health of course, is not a good game mechanic. That just won't happen. Jay Wilson has already talked multiple times about how the mechanics of Diablo II required massive one-hit kills in order to really endanger the players. Otherwise they had infinite health. Instead, Diablo III will have a more complex system to challenge the players, and will not include any sort of one-shot attacks.
That being said, the special death animation is an entirely different beast. Personally, I think the system is amazing. If you're going to die anyway, you might as well get your head bitten off in a fountain of blood. It started out as a on-off demonstration just for show in that first gameplay video. But the amount of positive feedback they have recieved has lead them to actually impliment the system in Diablo III where appropriate.