Beatty's eyes widen because he realizes Montag is defying him. Montag kills Beatty as an act of rebellion, not revenge.
When Montag toggles the safety on his flame thrower in Ray Bradbury's novel Fahrenheit 451, Beatty's eyes widen because he realizes that Montag is no longer under his control and is prepared to defy him. Montag kills Beatty as an act of self-defense and resistance against a dystopian society that suppresses independent thought and knowledge. It is not simply an act of revenge, but a symbol of Montag's rebellion against the oppressive system.