Schizophrenia and multiple personality order are two completely different mental conditions, just for you to know.

Yes, it is possible for a person to suffer from both, but it is rare. Multiple personality order in itself is a rare mental illness, schizophrenia being much more common. But enough of that.
I find it hard to imagine Peter going to the Dark Side. I can certainly picture his character becoming darker (it is already happening. He's not the innocent, mop-haired dreamer he used to be, even though he's still gullible enough to buy Adam's saving-the-world sales hype.), and doing some really bad choices, but becoming a villain in the strict meaning of the word? No.
The other two I can't picture switching sides are Hiro and Claire. Hiro's too noble for that (although his decision to bury Adam alive showed that he is not above cruelty and torture... More merciful decision would have been to kill Adam. But hey, even the best of the men have days off

) and Claire is too busy angsting about
unfairness of being different and the life in general to do anything noteworthy.
We've already seen "evil" side of Nathan in the first season before he decided to come into his senses and prevent the catastrophe, and Niki's Jessica personality was/is evil by default...
My point? Nobody of these five will become a permanent villain. The trailer is a red herring. :-P I see lots of morally gray ahead, all right. Of course, some may claim that resorting to evil deeds in the name of good automatically equals evil, but that's not me.