Diablo: a donut shaped universe?

Here’s an interesting video from the original Diablo that I forgot about. If you look closely at the guy laying on the floor after Diablo “dissolves” you can see that he did the same thing you end up doing – cramming the soul stone into his head. Which makes me wonder, why would you do it in the first place if that’s what you saw?
Do you do it because you think you are stronger? Or do you do it because you feel there’s no other choice? Or is the soul stone possessing you already, and thereby forcing you to do it?
It raises some interesting questions about just how many times a warrior has “saved” the world from evil, only to become that evil himself.
I wonder if they will address any of it in Diablo 3.