Maybe in Divinity 2 you play as that child (then grown), and have to choose between good and evil.
I think it's more the case of batteling the child.
But this brings the issue of the whole balancing the world back. Does evil magic create evil people just because it's evil.
I'd think that once the divine one turns grey and his power begins to weaken, demons will try to corrupt the dammed one. They couldn't do this before because the divine one's power was to strong. My guess they'll let a humen assasin kill the divine one when he is weak right before the dammed one's eyes. Anger over his slain father he unleashed his powers to kill the assasin.
[dramatic twist => see trough the eyes of the divine one how the dammed one fire a devistating blast towards the assasin]
divine one: no I have faile......d
[divine one dies]
the dammed one is search for vengance starts to develop his powers.
ofcourse in his quest for vengance the dammed one becomes corupted. His quest of vengence is steered by the black ring cult. When he realises what he has done it's to late.
well that's just my take. <img src="/ubbthreads/images/graemlins/alien.gif" alt="" />