- All of the people who Deckard Cain has ever mentored get turned into demons eventually. E.g. the archer at the start of D2 (forget her name). From what I've read, this is set to happen with the D2 heroes in D3.
- At the center of a number of crazy diabolical events, but mysteriously escapes unscathed repeatedly.
- Black magic: capable of identifying items without scrolls of identify.
- Among all the humans you meet in the field in D2 (except LoD), but two are alive. One is basically across a bridge from the starting area. One is Deckard Cain, SURROUNDED BY DEMONS, IN A CAGE. You're asking me to believe that the demons picked ONE PERSON to not kill, and it happened to be Deckard Cain? That's a bridge too far my friends.
Basically, Deckard Cain is this spirit who gets off on watching great men and women be corrupted. He's actually worse than Diablo, Mephisto, Baal, etc., in that he doesn't want to kill you outright. He instead wants to see you become what you hate. He's so powerful that he can even manipulate the prime evils to this end.
Well it seems obvious to me. This guy is bad news.