character realizes someone they trusted is actually evil, and act on it immediately.

starlord was about to join his father in assimilating the galaxy, before finding out that he was the one who killed his mother, and then immediately begins to shoot him. – guardians of the Galaxy

tessa is N's closest friend, the one who found him half-broken in a dump. but (in present time) tessa wanted to convince N to kill uzi, as the absolute solver was taking over her. "you knew about the patch (that would've let uzi get freed from the solver, letting her live), yes or no?" right as tessa was about to kill uzi, tessa doesn't give a proper awnser. "… do you know why i keep you around N-" and so N decapitates her. – murder drones