This is a humorous statement. Perhaps similar to the Irish proverb: "And may you be in heaven half an hour before the devil knows you're dead."

This is not a standard phrase. It is not a saying or an idiom. The meaning is not clear. I have to guess at it. It sounds like the kind of joke in which somebody makes a terrible threat that is so terrible that everybody understands it is a joke. To "nag" someone is to complain that they need to do something small. For example, if someone is supposed to take out the trash, and someone else keeps saying "you haven't taken out the trash yet," that is "nagging." It wouldn't justify killing someone. This is what my _guess_ as to what "tell God you died" means, but it's just a guess. If you kill someone, obviously you have committed a crime. In some US religious traditions, even if you are not caught and punished by the law, you might be punished by God in the afterlife. I think the speaker is saying "I'll kill you and I'll get away with it. Not only will I fool the law, I'll fool God." He is going to lie to God. He is going to "tell God you died," i.e. died naturally. God will believe him and not punish him. Obviously, this is not a joke that would be made by someone who takes religion and the afterlife seriously! It reminds me of a TV series called "Maude." Whenever someone did something that annoyed her, she would say "God'll get you for that." It is crazy exaggeration and overstatement, for the sake of a joke. In this kind of joking, sometimes you make it funnier by making more elaborate. My mother--just her own personal phrase--would say "If you do that again, I'll beat you with a rubber hose, because it doesn't! leave! marks!" It means "This really bothers me, it makes me want to beat you, but of course I won't." Added: I did a Google search on the phrase "kill you and tell God you died." It is more common than I thought. I just haven't heard it. All of the examples are obviously jokes. My wife has never heard this expression, either. Her interpretation of it is the same as mine.

Haha this is quite a funny statement. Adding the "and tell God you died" part almost implies that God doesn't care about the other person and/or that the speaker is on par with God.