Also, it provides a contrast with the leveling of Vinnie's temperment later in the film once he became fully enmeshed in the machinations of power politics and crime. That might very well have been Sonny's evolution had he lived.
"Temper like his father."
Sonny may very well have matured had he lived. People usually mellow out with age. But he was probably destined to die young anyway. You can glean from the novel that from when he was a kid all he ever wanted was to be a killer like his father. So, you know . . . live by the sword and all that.
