How did the Predators find an active serial killer on Earth?

Probably saw him make a kill. They are invisible when in stealth mode.

read, saw the local news reports

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A better question is why would they bother bringing him to their game planet? The humans are already at an extreme disadvantage and a serial killer poses no threat to the predators.

One ending under consideration would’ve had Dutch show up in an alien craft, suggesting that the whole thing was a test of sorts

While I’m glad they dumped it, it’s not too far-fetched to think that some humans may be cooperating with the aliens in exchange for technology or who knows what else. They track down killers and provide them as prey.

BTW, I personally don’t like to call the aliens “predators” because I like to think that title applies to the humans as well as the Yautja. They’re all predators, including and especially Edwin (Topher Grace’s character)

The same way they found everyone else. The certainly did extensive research on every single person they abducted. They probably looked up where a lot of people had gone missing and observed him in the act.