He kills the ones he doesn't deem right and takes the one he does. Basically he takes the one who is strongest or survives the longest. He had been watching the family for a while, being an exterminator they hired to help around their house so he knew the family and knew his way around the house. The traps were there to, well, trap the family so he could then kill them (spiders are shown a lot in this film which is symbolic; he's like a spider trapping it's prey in a web and then slowly killing them). We don't know who he was originally going to collect but one can assume it was one of the two daughters considering he had already started torturing the parents by the time Arkin arrived.
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