For me it has to be Terry Metcalf. i know you said not how the front office treated him but I have vivid recollections of Terry saying part of the reason he went to Canada was money, and part was he got tired of being booed for fumbling when he was the best all purpose player in the NFL at the time(rushing, receiving, returns).
Full disclosure he was and still is my all time favorite NFL player so I'm biased but I don't think it's a coincidence that he left and the franchise fell apart and went into a deep black hole for decades. He left, other players left, Coryell left in large part because he was furious the Cards didn't keep Metcalf. If you remember the story Metcalf played at Long Beach State in college and Coryell coached San Diego State. When they played in college Metcalf played so well against SDSU that Coryell remembered him and after getting the Cards job, he insisted they draft him. I read a book where Metcalf said after drafting him Bidwill and Coryell took him out to dinner, at dinner Bidwill asked him what he thought about playing CB and before a puzzled Metcalf could say a word, Coryell spoke up and said oh no Terry is my starting RB Mr Bidwill.
One of those things I've always wondered how the franchise would have differed if we keep Terry and keep Coryell.
