Actually, I don't think it's bizzarre. If you think it's normal that someone who is vegan will treat ordinary people as murderers because they do something that nobody else thinks of as "murder", except for the person who is vegan, then it's normal for others to treat vegans as a target for their (probably clumsy) philosophising. If we're gonna have a big old culture war, then it's a free for all and why should only one side get to have fun with it?

The root cause of this is an unwillingness to even acknowledge the existence of a different point of view in other peoples' heads. The militatnt vegan is incapable of considering the fact that "meat is murder" is her own opinion and that it is not shared by everyone around her, in fact she wants to make everyone around her not just aware of what she thinks, but also agree with her, and if people dont' agree with her it's because they're not just murderers, they're also idiots who can't see how they're murderers. The idiot who buttonholes you to tell you your ideas are wrong similarly can't get it around his big fat head that some people think different than he does- or he's just a fucking bully who thinks he's got the majority on his side and can torment you all he likes. I'm happy to hear your friends stick up for you.

Still, what happens to you is not that strange if you see it in the context of peoples' inability to consider others' point of view and understand others' thiking, even just to disagree with it better.

And btw, I think you and the other poster above are doing that too, when you say that others are "threatened" by your veganism. Perhaps it's just me, but it sounds more like you two are bragging about your unorthodox ideas that challenge others' misconceptions, than really trying to understand what's going on in other peoples' heads.