What to do if I am on a hiring committee and I am aware that a candidate has exhibited unethical behavior in his previous position?
If you're on the hiring committee, then your job is to evaluate the candidates.  I see no need to back away from that (and by the way, I can't see what your being a "foreigner" could possibly have to do with any of this).

One has to exercise some restraint in repeating "hearsay" in official matters: unless you can find evidence about the changed research, I would hesitate to bring it up because it's hard for everyone else to independently evaluate.  On the other hand, if you are a coauthor of a paper with the candidate in question, then your colleagues should be very interested to hear your take on that.  I think you should certainly describe the candidate's contributions -- or lack thereof -- honestly to your colleagues.  I might avoid putting the banner of "unethical behavior" on it: the ethics of it are really for your colleagues to evaluate as well, right?  In general I think that in these kinds of matters it is more collegial and also more convincing to provide information which is factual rather than subjective or purely evaluative.

There is also the issue of why you, as the first author of a paper, allowed a coauthor whom you feel did absolutely nothing.  Isn't that at least arguably unethical behavior on your part?  I think it's better not to open that can of worms, but rather say that in fact the candidate's contribution to the joint work was minimal and that you do not feel like he should get much credit for it.