Your friend isn't as developed in his moral reasoning as you are. He's at a stage of conventional reasoning in which he sees laws and social conventions as being synonymous with moral good, because they help maintain society and without them, things would fall into anarchy and chaos. Specifically, he sounds like he's in a subcategory of this stage called the "law and order orientation" which focuses heavily on obedience and punishment.

On the other hand, you're in a principled stage, which recognizes that people have their own different ethical principles, and it's up to them to come up with a social contract to get along. You seem to have the "individual rights orientation" within that stage.

Conflict is inevitable between these two stages, because you and your friend are reasoning in two entirely different ways. Any explanation you try to give to him will fly right past him, because he doesn't have the framework to understand it in a way that would make sense to him.

There's probably also some specific issue that caused him to go nuts over this in a game of D&D, but this is why your attempts to get through to him have been fruitless. Anything you say relies on the higher level of reasoning, to which he doesn't have access, so he reinterprets it on his lower level and then it just doesn't make any sense.