James Harrison (interestingly enough) put it best: "I don't want to injure anybody. There's a big difference between being hurt and being injured. You get hurt, you shake it off and come back the next series or the next game. I try to hurt people."
You want to hit people as hard as you can...you want to cause pain...you want them to hurt.
But you don't want to injure someone...no cheap shots to the knee that could put someone out for the season...no cheap shots to the head that could put someone out for their career.