Well it seemed like a logical way to go. Either kill him, which would be retribution for all the things he had done, or leave it in such a way that the audience could infer that he'd start his killing again somewhere else. That way, either it's done, or it let's us continue the story in our head. But what they did was just... why? It wasn't even a sort of salvation type of ending for him because he abandoned his own kid.