As for the coach, ownership, et al., I would need to know more about their role in this situation. My guess is that they created or allowed a culture where this sort of nonsense was allowed to happen. If that's the case, they need a punishment that will be effective as a deterrent for the heads of every other team.
I don't buy into the "oh it's just football" and "Martin needs to just man up" arguments. Yes, you need to be physically and mentally tough to play football and my guess is that since Martin made it that far, he is probably pretty tough. It must have just gone too far for him. I know the racial slur and the threat were too far for me. The fact that he didn't go to any of his coaches or other team leadership makes me think the treatment against him was endorsed by them so I don't blame him for leaving.
I think in sports like football, people don't often draw the line between being tough and being abusive. You don't have to be the latter to be the former. Incognito doesn't seem to have learned the difference.