[spoiler]... I really didn't buy that he wanted to give up his powers and live with her as a human[/spoiler]
On DS, love often does strange things to several of the supernatural characters. With some it influences nearly their every move, while with others it causes them to turn completely about face and to act in opposition to the way(s) they had been. Angelique would be the perfect example of the former, while Nicholas would be an example of the latter. We can and often do debate whether what they're feeling is actually love - but the characters themselves believe it is - and that's what's most important.
And with Nicholas the true irony is that he criticized Angelique up and down for clinging to her feelings of human love, but [spoiler]in the end it's very similar feelings that lead to his downfall in the Adam/Eve storyline.[/spoiler]
Actually, the influence of love or lack thereof is quite possibly the biggest theme running through all of DS because it affects each and every character in one way or another. In rare occasions it leads to happiness - but most often it leads to tragedy...