God, I probably spent 30 minutes on this wall of text. I hope in the end you will appreciate at least my efforts to explain the behavior of some people.

And please. It's not hard for me to even write this. Please. No need for aggression. If you don't understand what I wrote, ask again. Maybe I just described some things badly. You're still talking to the biggest fool who believed in "the best community among MMOs". So something like this)
You attempting to justify people's awful behavior doesn't satisfy my need for answers, in fact, you have NOT provided me a clear answer as to why you think tanks decide to grief players by sabotaging their runs because some DPS pulled ahead. You have given me nothing in this wall of text except on how you feel about your situation as tank -- you basically accept and are willing to spite people because you wish to counter your "feelings" of being worthless as a tank, nothing else.

I had to divide my answer in 3 parts because of character limit. Enjoy.