Here is my idea, if I were to "re-write" Rodriguez's script:
The Predators carefully watch and pick a select group of humans (murderers, assassins, soldiers, etc), after secretly, watching them in conflict and then capture the humans and take them to a "hunting" planet. A Predator safari. So, not only are the Predators dangerous, but also the group of humans. It's totally survival of the fittest. The "most dangerous game". Whoever is the last man or woman standing, gets their freedom and respect.
It's really another phase of the Predator training. The "winners" of the Alien pyramid, get their chance to hunt the most lethal humans around. These humans know about killing and death and are not afraid to do anything to stay alive...even if that means, killing each other.
Not only that, but the Predators are competing with each other. This is another test of survival. If they need to fight for trophies, they will. It's all about collecting as many human trophies, as possible. They don't work in a group, they hunt alone.
Now, as to how the Predators KNOW there is one human left, maybe the humans are given or implanted some kind of tracking device and the Predators can monitor when the tracking device stops. If there is only one tracking device left, then all of the remaining Predators must gather and allow the last human to survive a "final fight" with a chosen Predator and if the human wins the fight, they get to live, with the respect of the Predators.
So, yes, there would be the possiblity of Predators fighting each other for a human trophy. The object is to collect as many human trophies, as possible. Whoever gets enough trophies and survives, will face the remaining human in a deathmatch and if the Predator wins, they will move on to the next step of their training.
