The job was too big for him. I don't mean that we are massive, (though, of course, we are...) more that we were an absolute basket case at that point. Just relegated from the prem, divisions in the dressing room, senior pros who were about the same age as Murray who were never going to respect him, no recruitment strategy, the players he did bring in were shite, came into a strong league with Hibs and Rangers... he was just miles out his depth to the point that Alex Rae represented a brief improvement.
Obviously the above is the 'doing him some credit' view. Others will point out his terrible tactics and lack of leadership, etc. I think he was 34 when he took that job and it was just too much for him. It took a change of club ownership and, eventually, an inspired 18 months under Jack Ross to turn us around.