Answer: Revenge for killing Billy Bats
Tommy is shown entering a house, presumably for the "being-made" ceremony, when the family shoots him in the head from behind. This is because the family found out about Billy Bats' murder was committed by Tommy, and wanted revenge. When Jimmy Conway found out, he literally knocked over the phone booth next to him! Both Henry and Jimmy are angry, frustrated, and confused because having one of their own being made would mean they were basically made.