How do you not let the evil/hatred of the world get to you?

I find this very difficult. There are so many horrible things that are happening in this world (this could be because now that I am an adult I am seeing the world for what it truly is. But I don't know.). How do you not have a hatred for the people that are doing horrible things? For example the people who did the mass shooting in California earlier today. I know nothing of the people but I cannot for the life of me find a reason for these people not to be punished for what they did. Shooting and killing people for no reasons, let alone killing people with mental health issues, how can anyone justify it? How can you not hate someone who did this? I also heard today a little boy of 4 years old was walking with his mother down a street. When a pit-bull bit the boy, pulling him under a metal wire fence as 3 other pit-bulls bit and kill him while ripping him apart as his mother watches.

I know that these are extreme examples, but I just want to know how can you not have a burning hate for people that do horrible things? Sorry for my rambling