I think Dent works for the most part. The problem I have with these "people turn to evil" storylines is that there's always a brief moment, the one where they cross whatever line, where the the motivation seems to fall flat for me. I haven't watched Dark Knight since the theatre, so I can't remember all of Dent's fall from grace... but there was still a sort of "because the script says so" moment for me.

Someone here pointed out Hugh Jackman's character in The Prestige. That one actually didn't occur to me because it is so subtle. The more I think about it, the more I think it's a great example of someone giving up on all they once believed in after years of:

Escalating Competition
Obsession with a secret or goal
Promise of success over his enemy