You say it is just a game, but in an earlier post you referred to it as a job. When you have a job you have responsibilities and people depending on you. If you just walk away you can really hurt your coworkers and put the company in a bad position. This is the equivalent of walking away from your job without giving them 2 weeks notice where they can figure out how to replace you.
I guess my opinion would change drastically if it turned out that Martin was afraid for his life or something. But if he just decided it wasnt "making him happy" then I have a real problem with him just quitting and not giving the team a better chance to prepare for it. There are a ton of other players depending on him, along with an entire fanbase. The dependability aspect is the nature of the game, it comes along with being an NFL player, and thats something you are aware of when you take that NFL paycheck.
If I was being harassed by my co-workers and hated going to work everyday and nobody did anything to stop it, I would have no problem walking away with no notice.