I've read in the psychopathy literature that psychopaths (and remember that not all psychopaths are murderers) have empty eyes. From my abnormal psych courses, I would say this is, generally, true.
Well you wouldn't be able to tell with serial killers because thats how they got so good. Think of Ted Bundy nobody would have known he was a killer, he looked too nice and helpless to be one.
Yeah, I do think there's something to that. Some faces can appear to be downright diabolical and scary, and it could be due to the expression in the eyes. It is said that eyes are the windows to the soul, so, I guess you can take it from that. If you've ever seen a picture of Charles Manson, for example, there is evil incarnate.