Future Psycho on the Swings?

I've never taken it as a good sign when a child consistently has no expression on her face. For example, today at the playground, two girls around 8 or 9, definitely sisters and probably twins, were hogging the two baby swings. There was at least one of three regular swings available. I asked them to let me have one for E. They complied but then started twisting their baby swing and hanging on it. I requested that they stop so they didn't hit her. They did and one climbed into the swing and demanded that her sister push her. Then she made her twist the swing and let go and the whole time her face was expressionless. No sense of "whee, this is fun," nothing. And no recognition that she was doing what I'd asked her not to.

Her sister asked for a turn and she refused, said her turn wasn't long enough, the regular kid stuff. Clearly she was the dominant twin or sister because the other girl said nothing, just kept doing what she was told.

Later, when we were leaving the playground, I saw the same (dominant) sister stalking pigeons. It was creepy enough that I stood there for a few minutes, ignoring E's complaints. She had a stick or a thick piece of bark in her hand and she was slowly walking up behind the birds and throwing her missile at them. I couldn't tell if she hit any of them--they kept flying away--but she continued to follow them around the playground, again with no expression on her face. I looked around to see if her parent or anyone else was watching but I didn't see anyone. And I didn't see her sister either. She came close enough that I was able to ask, "Is your mom here? Does she know that you're throwing things at birds?"

She responded, "I like to chase them."

I said, "But you're not chasing them, are you? You're throwing things at them." She didn't say anything.

Now I am not a lover of pigeons. And I know they are disgusting birds. I saw a little kid running around, actually chasing pigeons and being delighted to see them fly away. But this girl was creepy and old enough to know better. If it had just been her annoying behavior on the swing, I would probably not have thought about her for another second. But add the pigeon stuff and I have to wonder--is she a future criminal psychopath?