Now take Jack’s words of wisdom and apply them to Will Parry. Lyra asks the alethiometer whether Will is a friend or an enemy. The alethiometer replies that he is a murderer. It doesn’t really answer the question but Lyra takes this as a sign that Will is a person she can trust. Put in context with what Jack Sparrow said, then a murderer can always be trusted to be a murderer. It’s the man that doesn’t kill that can’t be trusted. Now follow me here, since Will has already murdered once he can be trusted to do it again. Lyra knows that he’s not a coward now and that if they get in situation that involves killing in self defense that Will will go through with it. It’s like that old saying “Better the devil you know, than the devil you don’t.” Lyra knows what to expect from Will now rather than not knowing if her companion would be capable of killing. Making Will Parry the murderer you want on your side. Savy?