Why are some people so evil and horrible?

I was talking to my best friend on the phone before, and she told me casually that she was bashed a while ago at a train station. I was shocked, and asked her to tell me what happened. She said that four guys just attacked her, and held her down and bashed her. She's only 16, and very pretty, so I'm just grateful they didn't rape her (unless they did and she just didn't want to tell me). I got so angry and horrified I felt sick. To imagine this girl, so beautiful and young, brutally attacked by random people made me want to scream and hit something. I felt so horrible I didn't even know what to say to her. She also told me how her stepfather had been abusing her since she was ten. He'd push her and slap her and say horrible things to her, and when he overheard her telling her brother she was pregnant, he burst in and started bashing her, and hitting her stomach. She isn't pregnant anymore, because she had a miscarriage. I felt so angry I told her that if I had have known earlier, I would have killed her stepfather for that. She said he was an alcoholic and had a bad temper, that's why she didn't want to stay at her house, and why she left, and her mum doesn't want anything to do with it.

I just hate the world and people so much right now. To think that people can be so evil and horrible and disgusting makes me so angry and sick.