[It isn't like Rey has never killed children before. She's pretty sure she's killed a lot of kids when she went nuclear that one time, under certain special circumstances, even if it was unintentional. But she's since committed to the fact that she's not that person anymore. So, no murdering teenagers in cold blood for Rey.]

That's probably for the best. Better for her to be cautious and alive than careless and dead.

If there is a way to tell, then a pattern hasn't presented itself yet. As for why... [She scoffs.] Take a wild guess.