Why do people hate people?

We are all on the same boat we tend to get angry at someone no matter how bad or serious the issue is but, what is really awful is that we always seek revenge. I have, always been that way after I, do it I feel better then all of a sudden I feel guilty. All over the world everyone feels anger and it does no harm but it only does harm based on our mental state. I remember this past summer my ex and I went out had a blast then all of a sudden she hated me for something ridiculous. We had a disagreement she balled her eyes out I thought it would have been a good idea; that we y’all settle for the night. I got out of her vehicle; than all of a sudden I accidently hit her car door on the tree. She burst began hitting me I got out to look to make sure that (there weren’t any scratches) etc. She got out and took a look then she punched me saying I never hated someone in my whole life. A week later I texted her saying well it will happen this way you can hate me unfortunately your hatred doesn’t die with me it grows. So you run out into the night to hate another face and another and another. Until one horrible morning when you wake up you’ll realize that hatred has become your whole life and you won’t know why. After she replied saying that I was an, a**hole but she’s been battling demons of her own.