If you're a vegetarian for dietary reasons, you probably see meat-eating as similar to: smoking. It's a bad decision but it's only really hurting one person, so "just let them live their lives." This is how I treat actual smokers, or heavy people, or skinny people who drink a lot of Mountain Dew or whatever. I eat a lot of candy, whatevs.
If you're a vegetarian for moral reasons, you might see meat-eating as more comparable to: smoking, but also sometimes blowing the smoke into children's faces. It's easy to see how you'd probably feel justified in yelling at such people, or publicly shaming them or whatever. Maybe it would be more 'effective' to be really really nice and politely 'educate' them and stuff, sure. But if I really do morally reject the idea of eating animals or blowing smoke into children's faces, then it seems strange and unfair for someone to expect me to be calm and cordial about it while watching someone support it. I'm extremely morally opposed to it, remember?
What I am not saying here is that I, personally, verbally shame and destroy people who eat meat because of my ethical beliefs. (I only tend to do that to people who defend themselves with terrible condescending fallacious statements like "vegetarians are anti-science.") What I am saying is that, if someone does verbally shame and destroy people who eat meat, it is not really a reflection of their "bad choice in persuasive technique." It is only a reflection of them being really pissed off about something they really, honestly think is totally unacceptable, and you kind of shouldn't expect anything different.
It would sort of be like if someone came into the room and said something horribly sexist, and then I got up and screamed about how unacceptable that is and how much I hate them or something. Maybe that wouldn't be a very constructive way for me to educate them on gender issues. Okay. But it would be even less constructive, by a long shot, if some third party suddenly started undermining me based on nothing but my tone. Don't you think that would be strange, to criticize a message not for its content, but purely, 100% for the emotional output of its messenger? Isn't that the very definition of a "personal attack"?
Put me down as pro-"yelling at people for supporting things you are totally morally opposed to." I don't encourage it, but I do welcome it-- believe it or not, it's possible to participate in useful discourse while being offended and angry! Sarcasm and theatrics are fair game too.