//There are a few things but I feel like I already rant about them ad nauseam on this blog, LOL.  

1) His tenacity to survive, and how that dogged denial of the odds eternally stacked against him is a form of perverse heroism, and honor.  Most Masters seem to focus on his cruelty and his manipulativeness, which are qualities he’s learned to develop in order to maintain the upper hand in power dynamics, and things I definitely need to deal with as a writer…but I feel every canon muse is a matter of what parts of the canon resonate with you, and you choose to emphasize.  For me, Simm Master appeals initially because he’s so fucking defiant.   Even when it shoots him in the foot or (literally) stabs him in the back, he will not yield when his own sense of self mandates that it’s foolish.  He self-preserves, and that is not inherently evil. What it is, is inherently alive.  No matter what The Doctor Falls tried to prove.  

2) Closely connected, as many have more eloquently detailed, such as @natalunasans and @modernwizard, my Master uniquely emphasizes the neurodivergent roots of his hurtful behavior.  As a child, and in every face, his telepathic abilities were uniquely sensitive, and rendered his psyche, his feelings, thoughts, and impulses more porous than those of other Time Lords.  This porousness–a kind of sci-fi trope for Borderline Personality Disorder, Sensory Processing Disorder, and maybe something on the autism spec–was a source of social ridicule, and also of paranoia, and finally, most importantly, a thirst for autonomy. For self–mastery.  Never once was he certain his own thoughts and convictions were his own.  So he forced himself to fanatically shut off others, to deny any attempts, direct or indirect, to exert control over himself (a terrible irony is that from childhood Rassilon was already doing this via “the Drums”).