Can you be retarded (sub 70 IQ) and play pro ball? Did that guy literally cost himself $6M just being a jerk? Serious question. Could he just so completely lack capacity?
I think it was a pretty smart move. He would never be able to achieve anything with the Cavs, and his ego told him that he had to be the best player on that team (but neither his body nor his mind would allow that). Much easier to go to a contender, where he isn't even the best player at his position. That could eventually prolong his career several seasons.