It was a combination of dramatic effect and homage to "Bonnie and Clyde." The novel notes that the kick in the face was to put something "personal" in the killing.

BTW: The Thompson submachine guns used in that scene fired .45 ACP, a round designed for maximum knockdown power at close range. The first shot that hit Sonny would have disabled him. Also: if you watch the scene closely, you'll see bullet holes appear, disappear and appear again on the car's body.