I agree completely with TJ's comments and want to add that its not only current day gang bangers and low lifes that one should worry about.
The threat is going to come from a wide range that will include people who are now productive members of society with jobs and families. Starvation, post traumatic stress, the dehumanizing effects of a world turned upside down will do strange things to people. I have a pretty cynical view of humanity and don't believe it takes much for man to revert back to barbarism.
I wouldn't under estimate anyone.
I can see scenarios where families, friends and/or others band together in tribal groups taking what they need to survive.
History provides countless anecdotes for these types of behavior.
Finally, to TJ's point, there are likely those on this and other gun websites whose prep's don't go much further than their survival arsenal or a few months of food. What are they going to do when the food runs out and are watching their families slowly die from hunger, disease, malnutrition. I think they are first going to ask you, then beg you, and if all else fails fight and possibly kill you for food to keep from dying. The first kills may be emotionally difficult, but will be rationalized. They will blame the victims for not sharing or even if they did share, not sharing enough. They will believe that there family comes first and the victims deserved whatever happened to them.
I hope none of this ever happens. I am doing what I can to learn how to farm and produce my own food. I am trying to establish a sense of community with the small cluster of land owners that surround me. I am trying to build the infrastructure to support a group of family and friends to include some that I have met through AR15.com. Few, if any of these people prep, but I will take them in, because I am going to need them to help operate the farm and provide for our security and general welfare.
Think of it from the perspective of Mazlow's heirarchy of needs, the first level of need is physiological, food, water, shelter. The other levels range from level 2 which is security to level 5 which is self actualization. Most of us are currently at level 5, but if the shit hit the fan most of us will fall from level 5 to level 1 almost overnight. The rest of us will probably revert to level 2 (security) or level 3 (belonging, such as being part of a group of some sort capable of providing for level 1 and 2 needs.
Free fall of society is going to bring out the ugly side of man.
If the unthinkable ever happens, the world will have truly turned to shit. It will be medieval.