The first Predator's intentions weren't as noble as the humans thinkThe humans speculate that the first Predator stole the Predator-Killer suit and was bringing it to the humans to even the playing field against his own race. However this doesn't match up with his behavior during the course of the movie. The second he landed, he started killing humans, and during and after his escape from the Stargazer facility, he wantonly slaughters dozens of humans who crosses his path. This might be evidence that the Predator never intended for the suit to end up in the humans' hands and only came to Earth to hide it or himself.
- Borderlining on fridge horror, there's the possibility that he was trying to get the Predator-Killer suit for his clan so to have something to fight against the Ultimate Predators. And due to the fact he died on Earth and the suit is in human hands, that means that his clan is not only waiting for him to get back with a weapon that could help protect them but that they're currently losing their fight every minute the suit is not in their hands.
The Predators shown in the film were scientists.This explains why it's never mentioned anywhere else that the Predators genetically modify themselves. Notice how when the Autistic boy puts on the Predator's mask, he sees a vision of a struggling Predator being injected with a mutagen that gives him human DNA. These Predators were part of a clan, but got exiled for experimenting on their kind. The first Predator was one of their experiments that got away, and the Ultimate Predator was sent in to take him out.
Related to the above, the Predators in the film are "Bad Bloods"In the EU, Predators that break the honor codes or "Go Rogue" are called Bad Bloods. It's entirely possible that the Predators featured in the film are simply a group of Bad Bloods that got together due to their extreme methods and alterations of their kind.
The Predator civilization has experienced a drastic change in leadership.This could explain why the Predators are behaving so differently then in the previous films, even going against their own honor code.