For what it's worth, I watched the whole seasons 1 and 2, and I had no preconceived notions about any of the coaches. I thought the whole Tony Lucca situation made Adam look like a huge douche. Not that Christina was completely innocent, but when Adam and Tony kept insulting her (and calling it a "metaphor", which implied she was somehow imagining the whole thing), I actually felt sorry for her. That whole thing made me dislike Adam more than his treatment of any of the contestants.
I did think that artists like Katrina, Amber and Will were treated like red-headed stepchildren of Adam's team, and most of them definitely weren't among my favorites (only Will was, but by S5 I had long become used to Adam's favoritism, so his behavior towards Will actually didn't affect my opinion of him either way). The irony is that I think Adam's indifference towards them probably helped them get farther in the competition than they would have otherwise, so I wouldn't really say he screwed them.
Anyway, I guess my point is that any negative bias I have towards Adam now is based solely on having watched him on The Voice in the past seasons. It isn't based on any internet speculation or my positive bias towards someone else, only on what I have seen. Obviously two people can (and often do) watch the same thing and interpret it in a completely different way, but to me, Adam definitely doesn't come across as a very good coach or a particularly pleasant human being.