we don't know the story. Let's say it was a gang hit. He decides that if he kills their guy, the beef gets even worse and he and anyone he knows is marked. If lets the guy live, they might call it off in return for not offing their guy when he could have. Then he promptly gets the hell out in case the killer had backup.
Or idk maybe it was a chance encounter and he's not the killing type and just freaked out afterwards and wanted to get as far from that shit as possible without attracting any more attention.
Did anybody else's brain automatically add a fedora on top of all those people's heads?