I don't by out Keane is mental thing. We're talking about a man who shrewdly avoided sending an application to United because he thought he wasn't good enough to play there. Instead he went with a lesser club knowing that he could work his way up. That does sound a good plan of a well balanced and mature man. At Forest, he handled his transfer request only to be punched by Clough. A man man would have beaten Clough to a pulp. Instead Keane closed an eye to it, knowing that hitting a living legend would end his career as a footballer.
So, in my opinion, Keane knew exactly what he was doing. He picked the fights he knew he can win and stirred away from people who could ruin him. Take a look at his bully list. Heinze, Schmeichel, the Nevilles, Pally, Queroz all were all people with equal/less standing to him. He didn't try to mess around SAF, Cantona or Sir Bobby because he knew that he would have been shown the door. Which suggest that he was nothing but a big bully who wanted things done his way. He's a great player but not a great man let alone captain