I definitely understand your point. I personally think that she killed Caylee because she felt Caylee was a burden, and was preventing her from living the life she wanted- so she killed her, justified it in her own mind somehow, and moved on. Do I think she would just snatch up a neighbor's child and kill it? Not necessarily. It wouldn't serve her any purpose, as the child isn't her responsibility.
HOWEVER, the fact that (I believe) she killed her own child and got away with it might be strangely exhilarating to her (beating the system) and if she is impulsive and mentally unstable (and we all know she is) she might want to try to beat the system again somehow. Who knows what goes on in the minds of murderers...If she wanted attention or to "get one over on everyone" there's no telling what she would do. Getting away with something so big has got to make her feel invincible, and who knows if/how she might try to push those boundaries again.
But do I think she is just going to run around snatching up neighborhood kids? Nah, I personally don't. Would I be comfortable with her living next door? I wouldn't like it, but I wouldn't fear for my life. Wouldn't want my kids around her, though. Just my opinion though