know the game
I think the "bad" kind of violence that we should be considering is an emotion. A person can feel when a thirst for blood and vengence wells up inside. It's not good, and in my opinion, its not right.
I learned the difference by observing two completely contrasting football players in the NFL. One was Chicago Bears linebacker Dick Butkis. His game was based on fear, intimidation, and trying to take opposition players out of the game, literally.
Another was Oakland Raiders defensive lineman and Hall of Famer Howie Long. Although a great tackler and sacker of quarterbacks, he once said on an NFL Films interview that he "abhorred violence," and it showed in the way he played.
For him, it was a game of skill and finesse. He played the game the way I think that it was originally meant to be played. He didn't try to put quarterbacks in the hospital, he just brought them down to the ground and got the job done. The tackle was just the end result of his skill in getting there. The way he pulled down a quartback often looked like a father playfully wrestling with his young child.
I think that deep down inside, we all know the difference between playing the game and being violent.