Why do some vegans find it necessary to harshly judge non-vegans?

I want to know what goes into their beliefs. I find the extreme examples vegans to be smarmy a-holes and am really curious. Why did vegans threaten this girl for giving up veganism for health reasons? If you’re vegan and non-militant/psycho, try to make me feel better about your movement. As long as you don’t harshly judge me for what I eat, I won’t do the same to you.

This is your chance to calm my anger. GO!