I want to punch my 12 year old brother in the back of the head
Why would you harm your brother for no reason?
Oh, it's been a long time since I've wanted to punch him
That sounds like straight-up bullying. It's not appropriate to punch people let alone your own little brother. Siblings need to respect one another because sibling bonds are stronger than friends and having a good brotherly relationship is something many people take for granted.
He'll get over it. He's being a pain anyway.
Someone being a pain isn't a valid reason for you to beat on them.  If he's bugging you, tell him to quit it or even better just leave and go do something else.