I did something behind my ex bf's back and he doesn't know
Was this something from the past that affected you?
The way he fools around did, but I did it to get back at him for what he did to me
Its not a great idea to do bad things to your ex. Once this person is your ex, nobody is forcing you to even give them the time of day. Taking the time to do bad things to them just shows they are still on your mind.
He's a bad person, I'll hurt him and it's okay
You should not try to hurt him, that just means that you still care. It will be even worse if you do not do anything to your ex, that really shows him that you have moved on.