However these people don't understand that going out and killing someone for their food is a very risky business. It might be deadly even today. After the society collapses defenders will be taking intruders out at 500 yards. Armies have a way to deal with that; they have armored vehicles; but when not, they throw bodies at the problem. If you lose ten guys taking that position, it's just war. But very few criminals will be willing to die for a bit of food. If you are that desperate, shoot a bird; a sparrow probably has more calories than a can of soup anyway.
There is one more problem. Even if the criminal is willing to kill for that can of soup, and if he succeeds in killing someone, that is not a guarantee that the soup will be there. The resources will be limited. If one prepper has ten cans of soup and twenty attackers succeed in taking them, they only have ten cans for twenty people - and nothing else. They can kill everyone in the city, they still won't get food that is not there. That food will be all taken from the shelves of stores in weeks and months that precede the collapse. Preppers will actually notice that and escape the cities. Whoever is left are just as hungry as the attackers.
If we believe SciFi literature (and why not, writers are not idiots, and they thought about this subject) then the cities will be quickly divided into zones of ownership that various gangs will claim and control. Those zones come with food supplies (such as remaining stores, or a railway station, or a few trucks, etc.) All the food will be concentrated there, and it will be sold to others... if they can afford it. Everyone else will run out of their supplies within a month, and robbing them becomes pointless. If a gang is intent on getting food, it will not be rounding up grandmas. It will take control of a large food store or a warehouse. That's what makes sense. Adult criminals don't rob children of their lunch money - not always because they love children but just because children don't have enough money to even bother.
Also, those who plan on just winning their daily bread by using their warmaking skills will quickly find out that the ammo costs more than food. Ammo is a high-tech product; it does not grow on trees. But food does. You can grow food everywhere in this country; but there are only few ammo factories, and their production will be entirely under control of one armed faction or another. So those "thug preppers" can burn a few rounds on killing one guy and possibly find nothing of value at his site - or they can sell those rounds for a lot of food.
This is proven by history of humankind. How many prosperous, happy robbers walked the roads in middle ages? You could do it back then; soldiers of the local lord had no radios. Still, most robbers ended up hanging on trees along the road. It was a very high risk business. Robbers of 21st century will not have an easier life.
They are called “Occupy.”