"Its a maid duty to save its master" <- Reverses time to the point where the demon king lived, kills him and then goes back to her timeline. Its how it will end... probably.
Mighty maid magic manages to miraculously maintain master's mortality from the malice of the maladjusted manipulated maniac! A miracle most magnificent and our modest motivated maid might not even be mindful of the macabre moment menancing her master.
The saint maid sure made her young lady quite a fine battle armor... I mean ball gown. Just the sword trying to strike down the wrong first target has doomed it. Hopefully a maid won't have to go pick up her lady and clean up the hall in order to do so.
The slave guy's backstory is surprisingly really dark for a story as light-hearted as this. Pretty compelling though, I hope the demon lord gets expelled from his body and he gets a happy ending.