She happened to be in the middle of scolding Riley, due to her attempt on murdering Robby, which was unsuccessful, but yet, they were completely unaware of your presence...

Ruby: "—But that STILL doesn't mean you can attempt to stab him, Riley!"

Riley: "So? I don't care! I still attempted to do it, but you made it unsuccessful!"

Ruby was right though, it'd still be illegal since she attempted to murder, but that still doesn't stop Riley to try to argue back, but Riley knew Ruby was right too