What Solaufein feels guilty about later (killing Phaere) is actually a lot lighter than what he actually did (indirectly killed an entire House). How can he go from gloating over the death of the House to hand-wringing guilt over the death of one of the two most verifiably evil members of said House? I daresay it makes a difference that he once loved her, so it's plausible, but now that I look at it, it seems kind of...self-indulgent and ugly, actually.* It would be nice to have a dialogue option to say something to Solaufein like, "You were gloating over causing the downfall of the House! You know, better than I do, that when a drow house falls from grace it gets entirely wiped out, down to the children. Couldn't you, I don't know, find someone with a little less blood on her hands to feel guilty about killing?"
* The combination of, "Nyah-hah-hah, we're going to wipe out House Despana!" with, "Oh, woe is me, I killed Phaere!" seems self-indulgent and ugly. I have no trouble seeing Solaufein's complicity in destroying the House, on its own, as fully justified and compatible with being Good.