"It was payback for Billy Bats and a lot of other things, it was among the Italians, real (excuse me) greaseball sh--." Henry Hill (Ray Liotta) in "Goodfellas"

Remember, he'd tried to kill Billy Bats (with the help of Jimmy and Henry) in a bar during a fit of fury but he ended up still alive and he had to kill him in the car? Bats was a made member and Tommy wasn't and he didn't get the okay ahead of time to do it. They had to make an example of him.