Given the following context:  Andy, Trevor, and Billy are childhood friends. As young adults, Billy sells drugs and Andy does deliveries for him. One night, Trevor delivers instead and gets busted by narcs. 5 years later, Trevor is released from jail and learns his girlfriend Laura is now with Billy. Trevor gets back with her and decides to rob a local mobster.  Trevor wants some cash to buy Laura some gifts and Andy was told by some local mobsters that they are investing money with huge profits and he wants some cash to buy in. However, their robbery is very sloppy and the head mobster quickly figures out what happened.  However, he and Andy's grandfather were good friends, so he tells Andy and Trevor that they can work off what they owe by smuggling stolen diamonds. Billy finds out what is going on and is mildly amused by it until he realizes that Trevor has gotten back together with Laura behind his back.  He retaliates by convincing the mobsters that Trevor is a junkie and is stealing the diamonds that he is supposed to be delivering to supply his habit. He also convinces the diamond dealer that Trevor cannot be trusted with real diamonds and he gives him fake diamonds instead.  When he delivers them to the mob, they quickly spot them as fakes and assume Trevor switched them. Billy figured by doing this, the mobsters would send a goon to kill Trevor and with him out of the picture, he could have Laura back with no blood on his hands. However, since he was the one to notify the mobsters, they tell him if he wants Trevor dead he has to do the killing himself. He finds Trevor at Laura's house having just made love and kidnaps him at gunpoint. He takes Trevor to a field to kill him. Even though Trevor begs for his life and Billy struggles with his conscience for the briefest of moments, he pulls the trigger anyway, killing Trevor in cold blood.  answer the following question:  Who tells Billy he has to kill Trevor himself?
Answer:
the mobsters