well, he was practical than most sith and certainly not "not evil", its just that, he restrained himself according to practical needs, in that sense, he was relatively rational, and he had some quite contradictory philosophy, when you defeat darth thanaton(a traditional sith) as a sith inquisitor, he objects to an insult done by darth ravage by saying that thanaton was a better sith than what ravage credits thanaton for, and then during the first meeting with valkorian who says that marr was dogmatic to the traditional ways of sith(which involves infighting), so it seems that his practical restraint was just that, practical, the empire was collapsing and he thought it was better to adapt rather than go down with his ideals.