I doubt it’s a genuine belief that it will work, to be honest. People just love to feel superior. I’m convinced the hostile vegans have joined the cause simply because they found a club to make them feel superior over others in much the same way radically religious types use that cause to do the same. They believe with such vigor because they need to for their own sense of self-worth rather than because they actually believe in the cause. If they really believed in the cause then they’d be welcoming of new members. They’re not welcoming and downright hostile to new members because that would expand the in-group and shrink the out-group they get to feel superior over.

I assume they mean harm as in reinforcing the “snobbish” stereotype that vegans are known for, and OP’s screenshot demonstrates the same thing- potentially turning people away from considering the diet themselves.

You’re helping the planet and animals by not eating them, but if your snobbery turns 10 people off from adopting that diet, then I’d say you’ve hypothetically done more harm than good too lol