Why are some people in this world so cruel?

It's human nature. Humans are essentially apes with higher intelligence. Instead of that being a good combo, it is a bad one. Instead of them clawing and biting each other like animals, they bomb and shoot each other. Humans are a bastard creation and a sick breed. Unlike us manatees.

The Bible says it best, I think: Jeremiah 17:9 The heart is deceitful above all things, and desperately wicked; who can know it? There are people who are so damaged as children that they repeat the cycle with their own families because that's all they know. There are groups of people who are brainwashed into believing that this religion or that race are evil and should be destroyed. Some individuals like the idea of power and control, so they sabotage or backstab others just to prove that they can do it. And finally, there are people who simply choose to be evil and destructive. Their idea of fun is hurting someone else -- preferably the weak and innocent. They weren't abused as children or harmed as adults. . .they just have no conscience or compunctions.