You make good points, and I'm not sure why the questions you are being asked are so rigid.
If you grab the backpack, you get to take whatever it has in it. If you are there, you also can grab whatever is not in it. We don't know what was in it beforehand, and since it can carry additional items, we don't know what might have been added. At the same time, you can take and carry items NOT in the backpack without putting them in the backpack-- ie, an AK-47 could be carried in one hand (or slung over the shoulder, if there's a strap) and a backpack carried in the other (or worn on the back).
I can understand this easily. I have to think others who are objective and looking for truth can as well.
The FACT is, things were certainly taken. We don't know what they were. But it certainly opens the door for a gun to have been in the dead guy's possession (as Kelly says), and then taken away by one of the other invaders that day.