Here's another topic that used to be debated a lot on an old board I used to visit. Throughout the PS series, there is a recurring theme of people becoming evil by submitting to corruption, knowingly or otherwise. In PSIII, Siren, Lune, and Rulakir showed some evidence of this, but all of them eventually escaped control of the evil that held them. PSI's backstory says the same of Lashiec and the Governer is possessed in game. In PSIV, there is Zio's church, where people fell under his sway and were freed after his death.
But what about the other major villians, were they always evil or did they also fall into corruption after originally being good or neutral? Take the Profound Darkness, for instance. It's certainly as nasty as can be by the time it appears in PSIV. Just look at poor little Mile, the town that turned into a big hole. But, did it start out that evil or did it become warped after being imprisoned for so long?