Try talking to them first, asking questions and getting them to evaluate their own beliefs. Many people are indoctrinated and manipulated into a system that they haven’t really considered and would be better of rejecting. In that case, non-combatitive conversation can go a long way to enact actual positive change, rather than just virtue signaling to people who already agree with you.

If they’re not receptive to that, and still causing harm, it’s necessary to stop them via any means necessary. But still, try to avoid harming them as much as possible. As Milgram’s experiments demonstrated, most people can do horrific things when a trusted authority figure tells them to. Remember, they are still people, and sometimes people are influenced to do bad things.