So for the advocacy of slavery example, an acceptable use of violent reprisal would have to be directed at someone who is truly influential enough for their suggestion (or "suggestion," as the case may be,) to reasonably constitute an actual threat.
You do get that by juicing someone's face like a tomato so soon as they so much as sniff "We should enslave our fellow human beings." for freshness, no one ever gets the power and influence you are describing and for society that is a good thing?
The violence is pro-scoial and prophylactic.
Further I can't figure out what you think society gains by having people running around suggesting reprehensible things so long as they never get carried out.
You seem to think keeping a rabid animal in a petting zoo is a net positive, but as soon as it bites a few people boy howdy will it get a talking to.
It is more moral to use violence to coerce the safety and dignity of your fellow human beings than to force your fellow humans to weather the constant threat of enslavement so you can glorify whatever liberal Neutrality Morality deity you serve.