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.