I think that most folks who are "defending" Mario are saying that the situation is more complex than you make it seem.
First of all, Mario quitting on his team, his coaches etc etc etc is all on him. No question. It's his life, his career, his effort level. His personal responsibility.
But let's be clear, wins and losses aren't dictated by personal responsibility. They are dictated by performance. We've seen malcontents, quitters, folks of both poor football and normal character put out great performances with HC's who rein them in, cultivate their talent, and motivate them. Ask Randy Moss, perhaps the biggest quitter of all, and one of the best receivers of all time, especially with NE.
So when you show me a Head Coach in the NFL, whose claim to fame is 50% being a "player's coach" and having "his guys fight for him" whose first season is marred by a perennial ProBowl and All-Pro defensive player, who has no history of quitting, basically torpedoing his own career, I have to raise some eyebrows. I think that's fair. And personal responsibility or no, the Bills will be the losers when Mario is cut. Mario will lose too, but the circumstances that got us here will remain. And that may the worst thing of all.