Let's say he was going out for ice cream after lunch.
Vanilla, perhaps, was his favorite and he didn't want it to be sold out on such a hot day. So hot the road is sticky with melting tar. At home he has a wife and two kids who can't wait for him to bring it back, and they will eat it in the kitchen out of ceramic Mexican bowls with spoons. He will feed a bite to his wife and laugh, and the children will lick theirs out of their bowls at the table, so slowly, savoring every cool bite.
Or perhaps he was coming back with the ice cream in his car, in a rush, afraid it would melt. He has no wife or kids but wants to have the ice cream himself, out of a lonely flowered bowl in a lonely dark kitchen. It will be cooler in there than outside, and he will love the darkness and cold of it and his ice cream.
Maybe it isn't ice cream at all. Let's say he's late for his job. He works at a small department store nearby, selling shoes and outdated children's' clothing. When he is late his pay is docked, so he hurries quickly into his car and away, tying a tie as he drives and looking busily in his rearview mirror. One has to watch for coming cars.
Let's say he's the owner of a business and he was off on lunch break, now driving back to the city to work. It was a long lunch break, because his wife is at home with a new baby and he was so proud. He sat with them for a long time and closed his eyes. He fell asleep there in his house, holding his baby and his wife, and dreamed pleasant dreams until he awoke and had to run, goodbye honey, I'm off to the office.
Or he works at home, as a writer or an artist, and in a fit of writers block or a cramp in his hand he drove out, driving aimlessly, eyes unfocused as he drives along now, caring only about his muses.
Let's say he's just a man like any other man who hit a child and didn't stop.