Final answer:
Beatty's eyes widen in surprise or fear to Montag's defiance as he toggles the safety on his flamethrower, signifying Montag's rebellion and pivotal shift in character.
Step-by-step explanation:
Beatty's eyes probably widen as Montag toggles the safety on his flame thrower because it signals that Montag is making a decisive move against him, challenging his authority and the dogmatic system they live in. The widening of Beatty's eyes may reflect surprise, realization, or fear in response to Montag's unanticipated defiance. Montag's decision to kill Beatty can be interpreted as an act of rebellion against the oppressive society and his role in it, as well as a desperate attempt to escape the constraints that have been placed on him.
In the larger context of the novel 'Fahrenheit 451', this action signifies a critical turning point, showcasing the extent of Montag's transformation from a dutiful fireman to a person who fights against the very principles he once enforced.