Plenty of people see plenty of injustice and they don't go around writing manifestos and killing innocents.
He saw a cop being dirty. Poor him. He was picked on in school...poor him. It happens every day. Some people choose to try to make things better. Some people decide to make others pay for whatever went wrong in their life. Life is not fair to most.
There is NO excuse for his actions, ever, no matter what he's been through. The truth about it all lies somewhere in his statements and the official statements. I think he wanted to be something he couldn't have. Poor him. They were right all along not to keep him.