A plausible worst case scenario
I've struggled with this one. We've seen many examples in the movies or in books. They are stories meant to educate, but also to make money, and hence they are very dramatic since that sells. It's entertaining to watch a hero struggle with a scenario, find or rediscover love, and save the day. The reason is simple, we love to be entertained. It pulls away from our own problems albeit temporarily.
Aristotle felt drama was cathartic. We see their lives played out, it generates empathy, we put ourselves in the role, all the same hormones are generated, and we live through them vicariously. It's also a roller coaster, and the excitement, buildup, fear, and release is enjoyable to most.
A worst case scenario could be horrific. Any student of history can look up wars and their effects. Look up Fort Delaware or Andersonville, and see Man's inhumanity to Man. Look up great natural disasters in which there were large inefficient delays in relief teams' efforts. Research refugees like the Sudanese Lost Boys, and generations in refugee camps, starvation, fear, abuse, etc.
In the case of a global economic collapse, Americans could die in enormous numbers from malnutrition, hunger, disease, abuse, murder, suicide, drug abuse, accidents, lack of medicine, execution for theft, or simply a lack of preparation. If everyone in other countries has problems, who will come and help us? Most relief efforts come from the USA, Japan, Norway, Sweden, Netherlands, Italy, Canada, UK, France, and Germany. Other nations are certainly involved and have done heroic things like Russia and China but not in as high numbers. Most of those nations will be in a similar struggle for their survival. All of them may be.
I think Norway is in pretty good shape, but what do I know? Go Norway! Viking people are tough. Canada has such a low density in population, they likely are fine. They know how to live when it's cold. They have abundant natural resources. Not too great a growing season though. They did get a huge batch of radioactive fallout from Fukushima, which is tragic.
In the US cities, we could see a drop of 90% of citizens based upon a lack of carrying capacity and the transportation of supplies. If we look at all cities, that would be equivalent to 276 million deaths, a number so large, a quarter of a billion people, that it is staggering and unfathomable, and hence dismissed.
The reason we dismiss that number is that it seems impossibly high. Think rationally, not based on fear. Imagine a lack of food. This means it must be shipped in. From where? How will it be shipped? Who will distribute it? Besides those issues, the larger issue is that the problem is not localized. If it were, then we could rob Peter to pay Paul. We borrow supplies, people, and transportation vehicles to deliver it there. Whoever distributes the supplies must similarly be fed, housed, and cared for.
The worst doesn't usually happen. Take a breath. Whew. We're probably not extras in “The Road” waiting for our death-scenes.
That said, if the SHTF in a dramatic way, it could get medieval. Worse really. Having a little common sense, and preparing, and knowing some skills, being part of the solution, could save a lot of lives.
In the worst case scenario, too little is done too late. Riots break out, and cannot be handled by the local police. Military cannot be transported fast enough. Criminals take over and steal critical supplies before they can seized by local officials and the National Guard.
Soldiers stationed overseas may be stranded in a worse case scenario. They could get cut off from their means of getting home. Enemies might attack transports to prevent resupplying. Even in relatively calm locations, a lack of working infrastructure could mean an inability to simply feed them.
Missing a critical window of opportunity several times could spell doom.
Citizens will be on their own to maintain some semblance of order. Most have minimal defense capabilities, not anything that will allow a standoff to work for any length of time. Gangs of marauders easily penetrate households. Horrific deaths and worse occur by people without scruples or morals.
Fleeing citizens, separated from the meager supplies, die from lack of food and medicine mostly. If the weather is bad, then a lot will die from exposure since they've never slept outside. Some campers will survive, for a bit longer, and having some food, they might make it until that runs out. Many don't or won't hunt. Hunters will make it, that understand the need to trap. They'll save their weapons for protection, will probably band together, and I hope can maintain their humanity. Some of this group will die, as some accidents happen, marauders repelled, and food becomes scarce.
Migrations of people will occur as local carrying capacity will not support the hunter groups. They'll move away from the populated areas, and look for areas of fishing and game. Abandoned houses will be checked for goods to liberate.
The dead will be everywhere and unburied. The rot will draw insects based upon the weather and season. Disease will be transmitted though the vectors of insects, rats, birds, and into the water supply. Careless survivors will also pollute the water supply.
Rural citizens will be spared a little, but have their difficulty after a month or two. Clashes between hunter/gatherers from larger cities and local rural hunter/gatherers might very well happen. That is entirely plausible. Nervous shots could be fired as either group is unsure of the other's motives. On the prairie, many deaths occurred because of competition for food or perceived or actual invasion.
Some soldiers, officials, and a handful of citizens might band together. I wonder for how long? Many are married and have families. They'll be worried unless their loved ones are taken care of. Food has been stockpiled, but it will have to be distributed, and since chaos will be going on, dangerous to move. Central warehouses of supplies may sit undisturbed with a few guarding them, and no one coming to get them, and then, the wrong people coming to liberate them.
Late refugees, half starved, will attempt to cross into perceived areas of plenty, and die of disease, starvation, exposure, or a lack of knowledge. They hid in the larger cities, harvested undiscovered supplies from the homes of the dead, but once picked clean, have nervously dallied too long to make a migration.
The winners of any late battles may be the ones who have eaten in the last 48 hours. It may come down to who has the strength to repel another, not arms or tactics, as people will be so low on energy that they'll be unable to be effective.
Most wounded people will die. Anyone needing surgery will die. Many more pregnant mothers will die along with their babies. It will far more horrific than Hell Hieronymus Bosch dreamed up.