Amongst various theories trying to explain this "mystery" we also can find an oxygen supply failure.Another thing, is that his flight plan normally had to send him much more easterly of Marseille, in the Alps mountains near Annecy.
The suicide theory was also based on the fact that he knew this flight would have been his last war-time air mission. High ranked officers had decided to let him know about secret plan of Provence allied landing. This knowledge would have automatically removed him from flight schedule (risk of "talking" if beeing made captive)
It is true that his behaviour was controversed amongst the free French community as he didn't shared De gaulle's ideas (as many others !). But those mean spirits who declared him as a traitor usually didn't went missing in action but ended their days in bed instead.
It is true that no evidence of gun shots have been found on the wreck but very few of it have been recovered.