Or a bully?
Would you wonder if it was your fault?
How would your approach the subject with them?
Would you leave it be and hope they grow out of it?
How would you react when another parent complained to you about your child and his/her actions/words towards their child?
I know no matter how much I try to to instill fair behavior in my girls, one of them if not both will have a moment or two of nastiness. Last year DD1 was taunted by a boy in her class, he would steal her snack every day and break her pencils. While the situation was remedied it left a bad taste in her mouth. She's not a mean kid, she's the mama in the group. However I can see how all her niceness will eventually break. DD2... well... she's not exactly a nice little girl. She has a BS meter and she will treat people unkindly even when she knows she's wrong to do it. I've had to talk to her about it often. So in a sense I'm preparing myself for the day DD2 does something really mean and I get a phone call.