Bluntly, Perpentach's a sadistic monster who only releases people because it keeps things interesting. Keelyn looks pretty nice from her innocent apperance in the Grand Menagerie, and her 'pedia. Then Kael made the mistake of letting me write her text for Lord of the Balors...
The vampires have eaten enough souls in their times that the only way they could go good would be to commit suicide. Hyborem... yeah. Tebryn Arbandi is too cowardly to risk displeasing his dark lords, and Os-Gabella is competing with Perpentach for "Craziest Person on Erebus." Faeryl, meanwhile, is too paranoid to do anything kind for other people; she's convinced everyone is as conniving and self-serving as she is. I'm assuming Decius to be the Decius of the Calabim path, in which case he chose to fall and shows little interest in changing. Auric has too much of Mulcarn in him to change much at all. The leaders of the Clan of Embers and Doviello are looking the most angelic of the lot, and of them, Mahala seems the most willing to accept change and even good if it will serve her people.
EDIT: Actually, now that I think about it, and having read MC's ninja-post (I took a while!), Keelyn is moving up the rankings, although still below Mahala. She only gets the two moments of casual sadism,
She's clearly an improvement over Hyborem on the throne of Hell, although her motivations weren't exactly pure; she was avenging a slight, and would likely overthrow Sabathiel just as eagerly if he ended up in a summoning circle and refused to play tea party. Then again, Sabathiel seems like he'd take the opportunity and convert her, so...
Again, I'm sticking with Mahala, but only on a technicality. Keelyn is fundamentally selfish and spoiled. Any good she does is merely for her own gain. Mahala, meanwhile, is quite the opposite, genuinely concerned with the well-being of her people.
EDIT 2: And now I cannot get the image out of my head of Sabathiel, dolled up and willingly playing tea party with Keelyn and a Balor, occasionally rolling his eyes in a bemused "why am I doing this again?" kind of way.