You know, lathering up a sleek, powerful cherry-red automobile with his friends... getting wet, slippery... accidentally bumping hands by the rear fender, then purposely doing so.
He was already set up as a bad guy when he is described assaulting a woman. He is working on his car because it's something you do outside your house, and he had to be outside his house for Henry's girl to witness the beating, which is the whole point of this scene. I guess he could have been like watering his lawn, but you do that alone, and I think the best part of this is the dumbstruck reaction of his friends. The aim of this scene is not vicarious revenge fantasy but to illustrate the difference between Henry and normal, albeit shitty, people.