Why do people turn evil?

For 21 years (from birth to present). I was a good person, always wanting to help everyone. I would always try help people specially friends whenever I could. As kid in school I was the same, but it seemed like they would take advantage of me. I didn't care because I saw hope in the future...after all these are just kids, when I get older it will get better. I would always put other people in front of me and say, how would they feel if I were to (do something bad to them) or how they feel if they were I need (and I came through). Basically telling my self, I treat others the way I would like to be treated...that didn't work. Now I find myself emotionless, with a cold heart, and caring only for myself. I feel that church for example teaches us false principles that would apply to a perfect world..but this world is far being perfect. I'm not saying I have become a ruthless person..I'm just saying I don't care anymore. If someone needs something