Either I am going nuts remembering fake facts or I payed better attention to the movie.
When the guy stuck his head out it burned him for until he went back in. Later in his log Will stated that they were GETTING DUMBER because they were ignoring their own survival means to confront him. Throughout the movie there was no prooft that they were intelligent or civilized beyond living in groups. The traps were of the same type Will had made and even in the Wiki (which is locked form editing) it states he walked into his own trap. Im sure he has a lot of those all around the city to capture night stalkers for testing, he had a lot of pictures on the wall of ones that he had tried to cure after all. The things are dumb, they operate on the level of pack wolves at best, but they can't build and they can't lure him.