I've watched all four of those recent Christian Bale films (but not Newsies--sorry, Zoe, I know that disappoints you!). I'm sure I will also see his next venture as another dark knight of sorts in the fourth Terminator movie.
But while I know almost nothing about the man behind the literal and figurative Hollywood masks, I don't see how playing so many conflicted and disturbed characters could fail to affect one's behavior or seep into his soul. Indeed, the story of The Dark Knight (and most of the other films, too) is about the ubiquitous and alluring presence of evil, which seeks to make even the greatest fall.
Such darkness cannot be taken lightly, whether by the actor or audience. Neither are immune, and even just viewing some of these dark movies is enough to alter the psyche, at least temporarily. So I don't think it's unreasonable at all to assume that spending hours and months pouring into a role like Batman, or Heath Ledger's Joker, would have potentially a very profound effect on those who take on the part.
And it is a reminder that we should be cautious and guarded about the messages and scenes that we allow to infiltrate our minds. The Dark Knight is an incredible movie that can provoke important and thoughtful discussion, but it also depicts evil in a most vivid way. Whether that portrayal was responsible for the downfalls of its portrayers, I'm not sure, though it's hard to deny the power of the film medium.