People don't have a problem with tankies because they want to use violence against fascists. Violence against fascists is fine.
People dislike tankies because they're reactionary assholes. They dislike them for cheer-leading unjustifiable abuses and failures because they believe we're in some zero-sum game that excuses it. Because they're extremists, oppressive authoritarians, and want to use violence against all of their ideological enemies, including the "wrong kinds" of communist.
The good things they believe in and do aren't what people have an issue with: it's the inexcusably bad parts of their ideology people don't like, and the fact they're obnoxious about it.
Edit: I stand by what I said, but apparently its easier to recognize flags when you expand the image ... and my rant had nothing to do with the post.
The real version is the first guy says he won't bake a cake for a gay couple, which is shitty, but his right. Then the second guy comes along and says he wants to kill the first guy because nazis also wouldn't bake cakes for gay people, so the first guy must also be a nazi, and nazis all deserve to die, so this guy does too.
Then a rational person comes along and points out that both of them are being shitty and everyone loses their goddamned mind over it.