So as we know, all the Stunticons suffer from a mental disorder/illness. I see people dive into what Breakdown, Dead End, literally all the Stunticons suffer from; BUT MOTORMASTER. That's partly because I think people, inherently boil him down to an abuser and ignore that he mostly likely has Anti-social Personality Disorder.
Motormaster is often acting in his own interests, yes he might follow Megatron's order but that's because it favors the Stunticon himself. Any chance he gets to kill Optimus, is something he favors and wants. Outside of that, he shows a lack of care for anyone and everything else. That's not narcissism, if he was suffering from narcissism, he'd be playing the social game like Starscream does, trying to get others to favor him, over others, yet Motormaster doesn't care to play that game. Even sees it as a waste of time.
He doesn't care about anyone outside of his circle of Stunticons and high command, because, unlike the people he interacts with, they serve no purpose for him. Even then, he shows a lack of care to those above him in command, he most definitely doesn't care about rules if they don't serve him. Soundwave, Starscream, and even occasionally Megatron, struggle to get Motormaster to listen to them; all the Stunticons do, but Motormaster is actually the largest offender. And we all know he is abusive to his gestalt!
I'm not diagnosed, but I've done a fair amount of research and even watched videos of Anti-social people talking about their experiences with the disorder and how they try to cope and minimize their behaviors and Motormaster honestly fits the bill from what I've read and seen.
Please do not take this as a complete explanation of people with these disorders. Everyone is different, and experiences are different, especially when it comes to the world of mentalities.