Why does Moby not tell the police about Virgil's threats?

Moby does not tell the police about Virgil's threats for three reasons. The first is that if Sarah Byrnes finds out about the threats, then she will either go back home or run away; neither situation Moby finds acceptable. Second, if Moby goes to the police, then he is just giving Virgil another reason to come after him. Third, he promised Sarah not to tell.