The only reason to get mad at someone calling you a bad leftist is if you think it might be true. People will absolutely prey on your insecurity to make themselves feel like their chronic posting habit is praxis. The solution is to not be insecure. Be humble, do honest self-critique. Don't let lonely people work out their frustrations on you. There is a difference between pipe-lining and emotional blackmail.
Nowhere in her post did she call you anything even close to warranting the vile responses you gave her. At worst she said some vegan users (Didn't even mention your name or hint at you specifically) could be annoying in how they went about things.
That didn't warrant your very 1st reply to be "fuck you", or proceeding to call her a piece of shit multiple times. Any other user likely would've been at the very least reprimanded for hostility. You majorly overreacted.
When someone tells you that your advocacy for oppressed groups is "annoying," fuck you is an appropriate response. That's how I'd respond to anyone saying something similar about racism or feminism.
Justify your behavior all you want, you weren't being a hero to anyone but yourself.
Disengaging.