Quantum immortality sounds exactly like the mythical hell

And is about as real.

The idea of quantum suicide is that we should act as if we won't die, because there will be some future "us"es who don't die. But this argument is wrong because the person who makes the decisions isn't one of those future people - it's the present person, who is completely entitled to not want to die.

We can demonstrate that this fails even classically. Imagine a pill that, when taken, causes the taker to believe that taking the pill is a good idea. A rationalization drug. And suppose that this pill also causes violent diarrhea. Is taking this pill a good idea? The person who takes the pill would certainly say so! If one is to avoid similar entirely-classical silliness, one has to invalidate appeals to what future people will think as a reasonable way of deciding, and instead just make decisions according to the present person's preferences.

One response I've seen to this was "dying is not like a pill that modifies your judgement" - the trouble with this is that as modifying the output of your brain goes, putting a big hole through it is a highly effective example. Another possible response is that the present person is not "rationally allowed" to not want to die, for some philosophical reason. But that position is inconsistent with all our shiny theoretical models of what "rational choice" means, not to mention human behavior.

Ultimately, understanding quantum suicide boils down to understanding the difference between future utility and expected utility.