What's with all the hate toward mentors?

[Canadane]

I had a blm mentor that rushed ahead and pulled mobs. Tank said to let him pull instead. Mentor goes bananas in chat and proceeded to pull everything.
First boss fight, blm does a /sit the entire battle. Next boss, the mentor refused to enter the arena, just sat right outside the ring. He got kicked from the group. Then proceeded to whisper all of us how he was going to report us for abusing the kick system.
I cringe every time I see a crown. They tend to act like babies.

You seem to be noticing the bad ones while passing by the unnoticeable ones. There's a lot of mentors out there, and when one simply does his job (that is, plays the game, not strictly mentoring) you have not much to say about them. When one does bad, you associate it with an icon and are basically conforming to a confirmation bias.
You have every right to cringe but a couple of bad apples doesn't spoil the bunch. What you experienced was a bad player, not specifically a bad mentor. His mentor status has nothing to do with the situation.