The difference is that Martin did not actively go out of his house to kill someone. Someone came to him looking for trouble, and found it.

I know what follows isn't the point, but if the 2 burgulars had been in prison, they wouldn't have been free to enter hsi house in the first place. Prison should be so gruesome that you have to worry about where your next meal is coming from. It sure as hell shouldn't be the holiday it is now. If it served its purpose, those 2 guys would be so scared of going back, that Martin wouldn't have had to shoot him.