Why are kids mean to each other? What's the benefit to them?

My nephew (5) always looks after his little sister if no one's around (like when they are off playing at ChuckECheese, but he'll intentionally do things to make her mad or hurt her feelings other times. Or take a "job" off the board at school, just because it will make another kid sad or angry. This same boy will be sure to let the teacher know if a child hasn't gotten a turn at something, or is being left out of the group. What do kids want to do things that will hurt others?Why are kids mean to each other? What's the benefit to them?
Basically it's a maturity question. The ability to beforehand figure out how our actions will make others feel, and caring for the consequences, requires a certain amount of abstract thinking and empathic ability, and at 5 years of age he simply isn't mature enough to pull that off.

He probably thinks that his little sister looks funny when she get upset, and the fact that she's suffering from it hasn't entered his mind. Something similar probably goes for taking a "job" off the board - it's not like he wants to hurt someone else, but he sees some light entertainment for himself and is unable to foresee the consequences from another person's perspective.

Why he at other times will work for fairness is no conflict, it merely proves how trainable we are.
We're born human, but not humane.
Humane behavior is something we learn through upbringing and constant interaction with other people. Someone has probably explained the concept of fairness and equal treatment to him, and while he's too young to understand the ethics behind it he sure is old enough to understand the rules.
He'll quickly figure out that following the rules, or blowing the whistle on someone who isn't will get him praise and attention from the adult in charge, and maybe even some other benefits.Why are kids mean to each other? What's the benefit to them?
The Living Christ. Before you toss this idea out, When God got kicked out of school and out of kids lives they have the only person left, satan, there is no such thing as a vacuum if God is not there satan "will be" find out more free bible lessons www.amazingfacts.org God bless Christ centered TV www.3abn.org God bless.Why are kids mean to each other? What's the benefit to them?
Immaturity, plain and simple.Why are kids mean to each other? What's the benefit to them?
Same reason adults do.