Although, if you take the movie as stand alone, and internally consistent, then it may work.
The "little" predators have some sort of code. The "big" ones do not, apparently. Although I don't particularly think that predators would find serial killers to be adequate prey, maybe they just like hunting people who hunt people, no matter how they do it. But Topher Grace's character needed implicit trust in his victims, and gaining the trust of a predator seems to go against the nature of the movie and the hunters themselves.
What sort of hunter would trust its prey?
Perhaps I'm annoyed with this movie, because even though I know it's going to be a creature who kills with weapons, I always want something more... creature-y from my monsters. My first encounter with predators came in high school, when I asked my boyfriend to put on a monster movie. The only one he had was Predator. No Alien, no Jaws, not even Anaconda (which I dragged him to in theaters, and yes, we were the only ones there.)
While Predator was scary, in a sense, though infinitely more disgusting than anything else, it was a monster movie with weapons, and I felt cheated. Look at the thing's face, for crying out loud. Something like that needs a weapon?
I also felt, and have always felt, that humans were poor adversaries for predators. One shining exception in the movie was the scene where prey and predator fought with swords. The predator saw an adversary, put his gun down, and fought with his blade. Human guns, however, cannot outmatch the armor, or the weapons of a predator. In Predators, even a handful of grenades was inadequate.
It was nice to see Topher Grace though. And a movie with a female black-haired badass with a gun that isn't Michelle Rodriguez.