That makes sense, but if this player is the DM of a DND game, how are you going to kill him? Or is killing all the "PCs" enough?
I also don't really understand how Atou attacked the MC, is it Dominate Monster? I don't think Dominate Person would work since Atou, while being in humanoid shape, isn't a humanoid creature, right? And if it is Dominate Monster, then that's a pretty high-level party our MC is going up against.
And Atou failing the check is also a bit weird since, from all the DND stats, I would assume Wisdom would be her highest, but that's just me.
The reason is easy. You can read D&D rule that anything and monster can be an ally depending on the story and character.
Atou alignment is Evil and the witch is Evil...
This means the witch has an Alignment bonus in recruiting.
The problem with the previous attack that made Atou rebel is pretty simple based on the D&D rule.
1) They have not yet made Atou betray Takumi. They just recruited her and used her as a team member... This means Atou by D&D rule sees the witch as "Follow Takumi ally that needs help"
This happens a lot in D&D and Dungeon Master will be like
"Because you order Vampire A to kill his countess or his wife? He breaks up the party"
It never makes any sense in D&D rule to make an ally that did not swear betraying to attack their lord...
2) Like I said previously by RTS rule it was not possible to attack the leader anyway...
So there are so many impossibilities to make Atou kill Takumi at all. Unless they "cheating"