I'm no posek, i dont wanna say ne-thing wrong out here where people see-but I'm a man of action Let's break down scenario of your's-1)you found someone being machshil the rabbim
2)you were poseach bishalom with him and he won't listen
3)you have a mishna brura handy, see first page where he says with people like this there's mitzvah, after trying to negotiate toWAGE EMBLAZENED WAR WITH THESE PEOPLE
You can tell authorities, put up blurred picture of him around lightpoles, i don't know.
It could even be mutar to take his pc and run it over with a steamroller.
NOW I'M ANGRY!
He's killing people and your'e not sure-RISE TO THE OCASSION AND DO SOMETHING! ! THIS IS LIFE ,not a joke.
I had personal psak from rebbe in telshe chicago and in a diff. story from rebbe in tomo TO SEARCH AND DESTROY. rebbe said "....and if THE boy is upset that you threw out his mag., tell him that he can take you to a bais din. GET IT. nOW GO!