Because you've not snapped and gone criminally insane. Once you decide to be evil anything is possible.

It's the meaning behind the Jewish creation myth with the fall of man. The tree of knowledge opens your eyes and makes you like God, able to know both good and evil, whereas before if you cannot know what is evil you cannot imagine the evil act.

We say such things are unthinkable, but that is merely a lie we tell ourselves because we understand that the act is truly evil, and we are ashamed at our own ability to commit such atrocities we prefer to pretend that we cannot.

Something in this man's life convinced him to embrace evil. He was presented with the choice to trade his humanity in order to become a demon and he made the trade. Sold his soul for power. Or the feeling of power. The capability to commit evil was there but the soul is what stops you. Sell your human soul for power simply means giving up the one thing that keeps you from becoming an animal driven by emotion and base drives alone, once gone you gain the ability to do the unthinkable even to the point that you lose your own sense of self-preservation.