If you witness a bad interaction, if you're capable of taking action, inject yourself into the interaction in a calming manner. Don't take sides or accuse anyone yet, initially just say something like "we're all adults here, no need to raise our voices, what seems to be the problem?". Will this defuse the infuse the interaction 100% of the time? No, but odds are it will shorten the interaction, at least you did everything you could vs just sitting there and the crew greatly appreciates it. Also remember that someone who's berating the crew probably doesn't care about the rules and doesn't feel shame
While the premise of venting and treating the crew good was well intended, folks just need to realize that people can be "wired" differently and there are simply bad apples out there. There was just a passenger fight on a Regent ship (yes you read that correctly) where details were sketchy but the video shows 3 or 4 men who appear to be in law enforcement based on the techniques they're using, pin down another man