I don't know the best way to word this, but it keeps going around and around in my head so I want to try and put it into words... It's the tragedy of Dhawan!Master and his renunciation of his past as Missy (not to mention Lumiat) in favor of holding his self-hatred so tightly in his hand that it burns him, and stoking the fires of the rage and pain in his hearts which he actively feeds with every morsel of betrayal and rejection (real or perceived) by the Doctor.
"Redemption" doesn't get inherited automatically by regeneration—it has to be chosen fresh each time, and he keeps willfully declining every opportunity to start again.
ARG. There's so much more I want to say, such deeper meaning I want to imbue, such richer context I want to denote. And I just can't quite figure out how to sum it up in any satisfactory way. But I think the destructively horrifying (horrifyingly destructive?) nature of this incarnation of the Master speaks for itself when at the end of it all, they are both brought to ruin and the message he is most desperate to communicate is: "If I can't be the Doctor, neither can you."
DHAWAN!MASTER'S CRUELTY IS PROJECTION, REHEARSED OVER AND OVER AND READY TO BE AIMED AT WHOEVER (namely the Doctor) REMINDS HIM OF THE VULNERABILITY HE'S SPENT SO LONG PUNISHING HIMSELF FOR. You will NEVER make me believe he came before Missy. Ever.