It takes a very specific set of conditions to produce a child who can start fights, threaten or intimidate a peer ("Give me the jump rope or I'll kill you"), and actively inflict pain upon others.
Bullying causes a great deal of misery to others, and its effects on victims last for decades, perhaps even a lifetime.
The person hurt most by bullying is the bully himself, though that's not at first obvious, and the negative effects increase over time.
Most bullies have a downwardly spiraling course through life, their behavior interfering with learning, friendships, work, intimate relationships, income, and mental health.
Bullies turn into antisocial adults, and are far more likely than non aggressive kids to commit crimes, batter their wives, abuse their children—and produce another generation of bullies.
Oh yes, girls can be bullies too. The aggression of girls has been vastly underestimated because it takes a different form. It is a far more subtle and complex means of meanness than the overt physical aggression boys engage in.