When Beatty hit Montag, the "bullet" fell out of Montag's ear. When he finds the bullet, Beatty understands that someone else is an accomplice of Montag's, that someone else has been helping him out. When he realizes that, Beatty threatens to use the bullet to track down the accomplice (Faber) and arrest him too.
When Beatty says that, Montag has his first impulse to kill him. But his finger only twitches on the trigger. Beatty sees it and baits him into squeezing the trigger for real, and that is when Beatty dies.
In part three, "Burning Bright," Captain Beatty forces Montag to burn his own house down. While doing so, Montag has a two-way listening and speaking device in his ear that Faber created. Faber calls it a green bullet because of the color and for how small it is. It fits right inside Montag's ear canal. Right after the house is ablaze, Beatty is talking to Montag and asking him why he turned to books. Montag is listening to Faber, though, and Beatty catches on that something odd is going on with Montag. In an effort to discover what Montag is up to, Beatty "struck him a blow on the head that sent him reeling back" (118). The green bullet falls out of his ear and Beatty hears someone on the other end of it. He picks it up and realizes that Montag was in communication with someone else and not acting on his own accord. Beatty puts it in his pocket as evidence and says the following:
"Well--so there's more here than I thought. I saw you tilt your head, listening. First I thought you had a Seashell. But when you turned clever later, I wondered. We'll trace this and drop it on your friend" (118).
Consequently, Captain Beatty threatens to find out who Montag was speaking with and have him arrested. This places Faber in terrible trouble.