I don't care if he casually walked past him. Everyone thinks differently than someone else, and Im assuming that the guy thought it was best if he killed the kid. Maybe he was really pissed at him for trying to rob him and killed him, I don't know, but im sure he wasn't thinking as logically as you. Not everyone is calm or collected in a situation. And also, the casual walking may indicate that he truly was not in a right state of mind. Look it up, on certain occasions while acting like that in a crisis it signifies that a person may not have a clear understanding of what they are about to do. The kid committed an illegal act that was bound to get him in trouble, anyways. Robbing comes with many dangers, and that was one of them. It was a mere reproccusion. I don't care if the kid was unarmed, he committed a hostile act toward the man and he should have to pay for what he did. Maybe not with his life, but that was a possible consequence of what he did, and that happens to be what transpired. People should learn not to commit crime, because it always has a downside.
Also, even if the guy DID kill the kid, it shouldn't be his fault. None of that would have happened in the kids were smart enough in the first place to get a damned job or something instead of trying to steal from a store. The situation placed the man in an uncomfortable state of mind. You can't do that and think completely clearly (Well, not most people anyways).