You're right about the whole serial killer and starting off with animals thing. People who are cruel to animals are also cruel to humans.
I don't know this little girl's history enough to really speculate on her behavior and I hope she didn't get molested, but if she was purposely being cruel to the animal, then yeah that is a cry for help and attention.
I wouldn't say anything to your friend about you suspecting she was molested but ask questions if you feel that strongly about it. I don't know what I would do. Kids make mistakes in judgement... they don't know how much damage they could do to a small animal. It could have been an accident... or it might be more. There's no way of knowing just based on this... so in conclusion, I don't know.
RIP hamster