On that...
- During the discussion, Arthur points out that they were "Just kids..."
- During the scene on the train, he actually tries to diffuse what is happening while he is having a laughing fit. He only fires when his life is actually in danger.
While I guess he might have killed the kids, it felt like they were pointing away from that. Even later than that, he does not kill Gary in the apartment and even says that he was one of the only people who was ever good to him.