Let's say that it's the 23rd century. Climate change has gotten worse and resources are dwindling to dangerously low levels. Coastal cities have flooded, displacing hundreds of millions of people which has led to territorial conflicts and widespread unrest. Famine and drought are rampant. The major world powers fight over the last remaining resources.
Mass exoduses have already taken place, with the world's most powerful and richest corporations funding colonization missions to Mars and the satellites of Jupiter and Saturn. These societies are slowly becoming more self-sufficient, and their relationship and dependence on Earth continues to weaken as they essentially become their own sovereign states.
Now, this is where I need some help in thinking up a plausible post-apocalyptic scenario on Earth where the robots are the predators of humanity.
How about a world where robots have advanced to the point where they look like humans and have some level of sentience? They're still used as labour and for work, essentially, but they have gained enough intelligence where they want the same rights as humans. After all, they also have thoughts, feelings and emotions now. However, most major world powers refuse to give them that right, and a lot of the world still sees them as unnatural and wrong. Maybe one robot, and then a few, realize that they're stronger and smarter, so why should they take orders from humans? This leads to a series of events with conflict and violence between robots and humans, with the robots losing because they're vastly outnumbered. But maybe there's a human sympathizer that has access to a classified, catastrophically deadly bioweapon meant to be used in human vs human war in the most extreme circumstances, and he decides to off humanity and let his robot overlords take their place as the kings of the new world? The bioweapon wipes out a huge portion of the populace, and boom, now you have a level playing field where humans are the prey, and the robots hunt them down. After all, even in this new post-apocalyptic world, there are remnants of the same military and the government that wanted to suppress the bots in the first place. So the war between bot and man continues. The robots have decided that logically speaking, they cannot let any opposing forces live and grow into a population that can be a threat to them. So: either you're on their side or you're dead, your body to be used as organic matter for resources and raw materials.
Another scenario could be that we have robots, but they're more like the C3PO kind than the Blade Runner replicant kind. They have some level of intelligence but not exactly sentient. Still dangerous nonetheless. While mostly they're used for work and labour, they're also utilized as soldiers on the battlefield in the constant wars taking place all over the world. Maybe the scientists, while fine-tuning their systems to make them more effective killers, messed up somewhere and change their programming to make them kill ALL humans instead of specific targeted groups. So the robots turn on their creators as well, and this eventually leads to world-wide war. The aftermath is the same (let's say planet-busting bombs or somehow a bioweapon gets released again). A small portion of humanity left, without a lot of their technology and defenses, and the robots mercilessly hunting them down, following their cold, objective logic. The humans do have the worker robots on their side - let's say there are separate centralized AI banks that control each type of robot. The fight continues in the post-apocalyptic world between the killer robots and the humans and their companion robots
Which scenario makes more sense? And is there another scenario that could lead to a robot occupied world?
PS: As for where the colony worlds are in all this - they've just decided to nope out of the whole situation altogether. They have their own problems to worry about, just trying to survive in hostile, extreme conditions, and just can't take on a whole robot invasion, even if it means they lose out on the resources and materials they get from earth. They're on their way to become self-sufficient anyway.
But that doesn't mean the robots have forgotten about them.