I was in kindergarten when one of my classmates told me about how his dad physically abuses him and I did nothing about it
Why didn't you tell anyone about your friend's abuse?
I knew I should have told an adult but didn't do it because I was scared of getting punished.
It's understandable that you were scared and young and didn't know what to do. But now that you are older, you have the knowledge and the power to stop such abuse from happening. You don't want to repeat these experiences and just stand by letting kids get hurt.
That kid was arrogant, it's his own fault if he gets hurt. It's not like I was doing wrong in any way.
He was only a kid after all and a friend of yours. You should want to help out your friends. You can't change the past, but you should learn from it and be more proactive to help others when no one else will.