I hope he doesn't get in trouble for that! I for one appreciated that he let it slip.
Well that depends on if they had more than one gun. If they only had one, I guess they would have to do it one after the other. If so, what an unpleasant end seeing your lifelong best friend kill himself and then killing yourself with the same gun that was probably covered in his blood and stuff. I think they got their punishment for what they did. (Sorry if that's a gory description, but...hey, this is a murder spree, the details aren't going to be pleasant.)
I definitely think it was a mutual decision. I mean, realistically, what options did they have at that point? And if only one of them did it, then the other would be in this horrible environment completely alone, and probably wouldn't last long either. As I have said many times I think they were bound to each other, or in the street parlance, "ride or die" (literally). If it comes out that one of them was shot by the other, I'm going to think it's because the one was too scared to do it himself (ironic!) and asked the other to do it for him.