Here's a scenario. S punched V in the face. -- Z saw the whole thing and went and told others of what S did. What did Z accomplish? Nothing but make a bunch of people dislike S.
What should Z do instead? Talk to S when he's calm because if Z doesn't like what he saw, then he should tell him directly. And if S won't listen, then get someone he will listen to, to talk to him.
Of course, if he's dangerous, then people should be alerted. But if he's not usually like that and he is generally a good man, then it's best to talk to him or get someone else to. He might slip again once in awhile, but at least with this approach, we have a chance of getting something done instead of causing people to dislike someone and not getting anything done to fix the man's behavior.