At 10 kids have a pretty fair grasp of right and wrong, it's the consequences they may not fully understand. I think at 10 they do know killing another person is wrong, and that when someone is dead they are gone forever and can't come back.
Kids will dislike other kids for really shallow stupid reasons such as how the person dresses, talks, or looks. The girl could have done something, or she may not have. She may have been one someone who just didnt fit in.
Bullying now is different than it was 15-20 years ago. It's more than just name calling and the occasional fist fight. Bully now is constant and vicious, via cell phone and internet. Fights involve getting stomped on in the face, broken bones, and more.
I'm just not a believer in excusing childrens behavior with "kids will be kids". All this mamby pamby political correct parenting, not spanking, everyones a winner, lets not dare tell the child no and bruise their ego is what has gotten us to this point. I don't believe in beating children, but some discipline and the occasional swat on the rear end does some good.