Why is Sonny Corleone killed?

Sonny Corleone is killed due to a combination of his impulsive nature and a betrayal within the family. After his father, Vito Corleone, is nearly assassinated, Sonny takes a reckless approach to retaliate against rival gangsters, exposing himself to danger. His aggressive actions make him a target, and he is ambushed at a toll booth by the Barzini family, who exploit his lack of caution. This assassination is a strategic move to weaken the Corleone family by eliminating its most volatile and aggressive member.