This was so unnecessary. The way he wanted to stop her. First he think at the beginning of the manager that demons are the worst of the worst. They ve kill everyone that had deals with them except for the mask guy. Not that he didn't tried. She's been hunting them for many chapter and not a peep from him.
Now her brother make a pact, gets a pretty high position, kill his own mom and gave her the head. She decide to kill him and all the other demon servant.
And hes like "this is wrong, gotta stop her" like dude, her brother deserves to die. You should've let her kill them, then try to calm her in a positive way. Not attack her first try to talk later.
This was stupid.