Bit of inconsistency I noticed in the translation. On page 30 of this chapter, Alice says "master, please watch your footing as well", but on page 15 of chapter 30, the snippet Luna heard included the word "properly".
Came back to check on this why is he straight up trying to kill her over her wanting to be Tias big sis lol also isnt him doing this and joining hands with the hero basically betraying the demons and Tia to begin with
Because her idea of being Tia's big sister involves turning Tia (and Ash, and everyone else) into subordinate vampires. If he can turn Mistos into his own subordinate first, it would solve the problem, but that's a big if, given the difference in both personal strength and army size. Assassination was definitely the safer bet.
And Ash teaming up with the hero isn't really a betrayal because he's working toward the eventual goal of turning the hero into his own subordinate.