... something strange. What should be happening is you can't be killed while the effect is up (your HP will not drop below 1; can't quite recall what happens if you trigger it when you're already using die_at, but I want to say it just brings you up to 1), and then when it ends you're healed for the amount noted. That's all it does, though; effects will still latch on to you, DoTs will still be applied, etc., etc., etc. If you're not killing during the effect, you're going to end up with 1 HP and quite likely die immediately to some lingering effect. If somehow you've got a +max HP effect that's happening to wear off before you can heal sufficiently... you'll probably die (this actually used to happen to me when I was first using the talent, iirc, since it was on a berserker and Bloodbath was wearing off around the same time... which on a low murder activation, meant I fell over dead at the end), though I'm not 100% sure on that. May have been changed behavior at some point. I am pretty sure (but not entirely, as it's been a while since I've used the talent) if it's stripped from you mid-duration, the heal doesn't trigger (i.e. you're at whatever amount of health you had, probably 1, and are likely going to die), though. It's a freakishly powerful talent, but it's also a pretty risky one.
Now that I think about, what sort of death message is it giving you? That would be your biggest hint as to what exactly is going wrong.