Reddit reviews Forever After

> I can't wrap my head around the logic of Tyrael's actions in the trailer.

Someone said that according to the lore his sword can only harm evil things. So everything is pretty logical, he intends and succeeds at sticking the sharp end into the death dude, it's then that he has a totally unexpected problem with the dude turning out to not being evil at all. That's why he asks "why", by the way.

And if you are wondering what's up with hiding the soulstone and using ordinary humans to guard it, that's actually a pretty established trope: you can't store powerful (and especially evil) artifacts at your place because they do bad things to the fabric of reality, you don't want to destroy them forever either because what if you need them again (and in this case, destroying the soulstone would let Diablo to be reborn back in the hell), so you have to hide them in some pretty inaccessible and remote place, preferably lacking in powerful wizards and the like. There's a pretty funny book by Roger Zelazny and some other people entirely built around this idea.