Billy decides to make his final stand. Like he said earlier "we're all gonna die," so he strips to his waist and cuts his chest in a ritualistic way, ready to die. Later, all we hear is Billy screaming in the distance. Billy was a badass and for him to be screaming like that, it made the scene all the more terrifying. The best part comes soon after when we see the Predator rip out his spine and keep his skull as a trophy.