Well, to his defense and not excusing all the evil deeds he made.
After all if you go to bed with the Devil, you better know there's no way your soul will be free.
Ketheric despised his son (thiebold, i think according to dialogue, let's hope not) for being a fool and drunk, sure a disappointment to every family
Isobel was his hope for the Thorm family, the girl was talented, smart and strong. I don't give a damn about her sexual orientations, love has no face.
when she died, Ketheric lost his way, he pratically sell his soul for her and to keep that bargain he had to move from everything he believed before, and when Melaine, his wife died. He losted even more.
I find his story sad because, he lost everything was dear for him and when the good gods turn their back on a devoted follower he got angry as most people would. So he did out of despertion to bring back everything and made a mess out of it.
When you fight him on Moonrise, he thought himself invecible for the powers he got in exchange for his power. He did bad deeds, yes, but you can see clearly at the end, he regreted everyhting he had done wrong when his daughter brought back to life shunned him out.
But you can see there's not lost love between them, her room has his bust as selune paladin or follower, whatever.
When he died, she wasn't happy for his death, she was happy that whatever took hold of his mind was stopped. The only one happy was Jaheira and Aylin.
Isobel clearly says in camp: "May the gods rest his soul and he finds peace", there isn't hate in there, just pity for how low her father fell.
