Scream enough, and they will start to sound like you. They will actually lose the ability to sound like they did before, and their insides will readjust to seem more human. Their genetic codes seem far too unpredictable to be engineered, which suggests that these things have evolved naturally. If you stop screaming, and talk to them instead, they will parrot you. If you sing, they will learn to sing. There is no advantage to hunting like this, they only reveal this after the capture. For all intents and purposes, it is psychological. They are justifying the hunt to themselves.