We don't know that Mag was pissed off about anything. For all we know, Mag was simply and calmly being intelligent about his future. Might not have been happy with the situation he was in, but I'd guess it was more due to injuries and earning potential than anything else.
I haven't seen any evidence to suggest that Mag did anything unethical here. Also no evidence that he should have "gutted out the season". I only see fan speculation based on one Pike comment about being medically cleared.
No player should ever do anything because of what the fanbase thinks. No offense to us all, but screw the fanbase.