“We’re all capable of the most incredible change.We can evolve while still staying true to who we are. We can honor who we’ve been and choose who we want to be next.” –the Thirteenth Doctor, Series Eleven Episode 1, The Woman Who Fell To Earth.  ❤ 

That aside, it’s not at ALL a dumb question. It’s one I grappled with a lot after The Doctor Falls aired. It was truly devastating to me to watch character regression of that magnitude, after End of Time.  But I don’t believe that it’s evidence that the Master is actually “a worse person” than he appeared to be during the RTD era so much as it’s a difference of how to frame his character.  RTD liked to use dark whimsey, and Moffat was more into flashy grimdark dramatics that translated less well from a previous era’s Master (he also pushed, perhaps a little too obviously, and forcedly, Missy’s redemption at ANY prior Master’s expense, when I always thought Missy’s evolution as a person was obvious without throwing any other Masters under the bus).  

Additionally canon hints  that during the briefly mentioned period during which the Master was on Gallifrey, Rassilon certainly didn’t let him get away with assaulting him with intent to kill, without some kind of punishment, likely both physical and mental. And by mental I mean involving Time Lord psychic energy, memory erasure, and distorted memories and perceptions. Rassilon and the Time Council deeply feared and loathed the Doctor and the Master and were keen on maintaining what Rassilon himself referred to as the “Enmity of the Ages” between them, because they have long been the most notorious rebel outcasts of Gallifrey’s Great Houses.  The Master referred to his exile from Gallifrey as a “mutual kicking-me-out” which confirms this notion that friction existed between himself and the Time Council, probably after they conducted all kinds of memory distortions to delete his memory of the truce he and the Doctor arrived at.  

I also believe that Simm Master doesn’t want to “Stand with” the Doctor not for wholly selfish reasons, but also because he has come to see doing so distortedly (and sometimes with the aid of the Doctor’s own actions, with Missy) as equivalent to total surrender.  To losing who he is completely, and to becoming a fill-in replica Doctor.  This is because his mind is highly sensitive and permeable (he has always canonically proven to be especially gifted at Time Lord telepathy–all those “you will obey me!” sequences in Classic Who) to the will of others and he has had to force himself to block out the person who has always mattered to him most (The Doctor).  Some of these ideas are not just my own.  @natalunasans can point you to her friends who brought this up incredibly articulately, and @trionrevolutonary has some of their own theories as well.  You might also talk to @alez-on-mars and  @doctamastacanon  who share a LOT of my qualms with The Doctor Falls.  

Primarily, I believe this was a case of clumsily handing off a villain from a VERY different past era of the show to a present showrunner, for the sake of the novelty of a double-Master episode, and while the idea was wonderful, the execution was just slightly off, and nobody suffered for it more than Simm’s Master.  

He was largely in character, in terms of details, but in terms of the larger character, it felt very much like someone replicating an old painting, but with an old photocopy of the original that made all the colors of the replication slightly inaccurate. 

So no, in the end? My opinion is unchanged. If anything because Missy comes AFTER Simm Master, the germ of her existence lies directly within him, he gives birth to her, meaning that potential for good lies within him (also remember Professor Yana!) and MISSY changed for the better.  Simm Master just needed 70 years or more of the Doctor’s undivided attention and patient care, too.  We become what we believe we are. We believe we are what we are told we are.