Violence, by definition, is an unjustified use of force. If a use of force is justified then it isn't violence.
For example, suppose you're walking across a bridge and you see someone about to jump to their death. So you run over, pull them back from the brink, knock them down, and sit on them. Have you committed an act of violence? I would say not.
On the other hand, suppose the person is just standing on a street corner waiting for the light to change. If you run over, pull them back from the curb, knock them down, and sit on them, that would in fact be an act of violence.
That's just a rhetorical device. I'm not suggesting that word definitions are prescriptive.