So the Master returned to Gallifrey and uncovered something he considered so bad that he burned the entire planet. What on Earth could have been that bad?
now don’t get me wrong here the Master has absolutely no love for Gallifrey or his fellow Time Lords. He has put the planet in danger numerous times for his own selfish reasons. So the knowledge that he decided to turn his homeworld into something resembling Australia at this time, was not shocking. That he did so without having anything to gain from the act was surprising. Of course that raises the question of why he went back in the first place? The Master never does that sort of thing without a sinister reason.
And still there is the question of where does this Master come in relation to Missy? The obvious answer would be that he comes after Missy who somehow managed to survive being shot. But there’s also the possibility that Missy did not follow immediately after the Simm Master. Perhaps there is a regeneration in between. There could actually be a number of regenerations in between and Missy acts as the Master’s final regeneration many incarnations away.