KITSUNEKIN FLAG🎋

Him identifying as Evil is all he has left. Not only did his last regeneration prove they were capable of a genuine act of selflessness, which would have shaken his sense of self identity anyway and led to some serious soul searching, but they then Died for it in a moment of genuine self inflicted brutality (twice). This one was born of themself rejecting, violently, any idea of goodness. While at the same time, having just proven the Doctor can Genuinely Affect them enough that through the space of just one regeneration they can go from Simm to Missy. 

So the Doctor is the cause of their loss of a secure Evil Identity due to what happened in season ten (even if the trauma around that isn’t the doctor’s fault). 

Then the master goes messing about in the matrix and finds out things that he wishes he never had, a master who just lived through finding out that the doctor could change them so deeply, suddenly finds out that not only is the doctor proverbially in his Head changing him, they’re also, literally, a part of him. A part of him that changes him, that he can’t get rid of or erase. The doctor is a part of his genetic code. 

So the master who just seriously traumatised themselves by murdering themselves Because Of The Doctor Changing Them... Then gets dumped with the knowledge that the doctor is capable of, and has, Literally Changed Them Genetically.

This time lord? Has No sense of identity left beyond being Evil. Like, one of the very few moments he was Genuinely being honest in this episode, imo, was when he was saying it killed him that her genetics were a part of him. A part of him that changed him over and over. Because that’s Literally just salt in the wounds of how his last two regenerations died traumatically. 

All he has left is his evil self identity, he’s not going to give it up to help her or ever admit he’s more than just Evil. This whole fiasco robbed him of his identity as a time lord and his identity as childhood friend of the doctor, his very history both personally and in a wider species context has been taken from him.