The fine line between child and creep.

I’m having a problem lately getting Pike, my antagonist in Fie Eoin, to be more creepy and less like a little boy trying to flirt but not knowing how.  Granted, the in the beginning of the book he has just gone through the ceremony to become a man, so he probably should act a  bit more like an immature boy, but just six months later he’s trying to kill people (actually, it’s much sooner than six months, but that’s when it really gets bad).  I’d like for the jump from child to serial killer to be a little less pronounced, and so I’d like to give him a good creep-factor at the beginning of the book.

So far all I’ve managed is immature brat.  I know what creepy people feel like to me, of course, but trying to put that creep-factor into words, and trying to make a character exude that, is difficult.  I’ve done it before, but the characters that I’ve done it with before are much older, and the situations much worse, and I just don’t feel like Fie Eoin needs a rape scene.  Not even a fade-to-black one (I don’t write rape scenes – it’s one of those things I just can’t bring myself to do).

Do you have any suggestions for creepy younger characters?  Do you have a character type that you just can’t seem to pull off?