It is not Callista's fault that Alexander has become such a diabolically evil man. It's to be blamed on a group of mysterious and powerful women under whose heel Alexander himself has fallen...
Also, I wouldn't be so quick to whitewash Alexander by making him out to be a one-woman romantic in love who adored Callista. This evil "romantic" fucked everything that moved and fucked a bunch of different women who bore him daughters. Yes, Callista was his true love, but what was Callista's fault if the first child he and Alexander had (apparently) died at birth? The guy's brains went crazy, and he found nothing better to do than try to fuck Elena too in order to get his coveted son...And he ended up just cheating on Callista, who caught them both having sex...So any way you look at it, Alexander's idiotic attempts to play eugenics and "breed a new, perfect heir" have only himself to blame.
But of course Callista has a lot of questions to give us answers to, and here I agree with what
MilesEdgeworth writes, and we'll need more than one flashback and more than one heart-to-heart talk (between Kane and Callista) to draw conclusions about all of this and evaluate Callista's role in the story.