Scythe Faraday writes in his journal that he prays every day. He doesn't know who or what he's praying to, but he asks for guidance and that he never become desensitized to gleaning. He says that he wants all of humanity to feel it when they witness death. Empathy is what will keep people human, and no god can fix it if humans lose their empathy.
What Faraday suggests here is empathy is both a natural part of being human, as well as a choice that people must repeatedly make if they wish to continue to be human. He suggests that losing empathy represents the point of no return.
Rowan finds Goddard in the chapel with just the curate alive. Goddard brightly tells Rowan that he can glean the curate. The curate spits that this massacre will help the Tonists' cause, and Rowan refuses to glean him. Rowan says that Volta self-gleaned, and Goddard knocks the curate out. Goddard says that Volta was weak and tells Rowan to take Volta's place, but Rowan refuses this offer as well. Goddard looks so deeply into Rowan's eyes that Rowan feels violated. He says that he knows Rowan loves killing and points out that scythes are called scythes because they're the weapons of mankind. He encourages Rowan to glean the curate and ignores Rowan when Rowan calls him a monster.
Goddard's lack of emotion for Volta makes it even more apparent to Rowan that he's an evil and heartless villain. This tells Rowan that if something were to happen to him, Goddard won't be there for him like Curie was for Citra, or like Faraday tried to be for both Rowan and Citra. Goddard is both a horrendous mentor and a horrendous person, which makes Rowan feel as though he has no choice but to start speaking the truth and take Goddard out.
Rowan feels dizzy from the smell of the blood and the smoke, and he almost talks himself into gleaning the curate. He pulls out his sword, steps forward, and realizes that Goddard always had him leave one person alive in his training to prepare him for real gleaning, when he can leave nobody alive. Rowan leaps forward and stabs Goddard. He tells Goddard that Goddard made him this way and rips his ring off. Goddard tries to speak, but Rowan beheads him. He fights with Chomsky and Rand and kills them both. The curate finally stops Rowan from bludgeoning Chomsky and offers to hide him from the Scythedom, but Rowan asks the curate to find survivors and get out. Rowan starts shooting off the flamethrower.
Violently killing Goddard like this allows Rowan to finally draw on all the things that Goddard taught him—but ostensibly, for a purpose that in the long run will serve humanity and better the Scythedom. This reminds the reader that while violence and cruelty may be inherent to human nature, just like empathy and kindness, it's a choice to draw on any one of those things and put it to work for a certain purpose. This, the novel suggests, is what makes humans human.
Rowan steps outside wearing Goddard's robe and ring. He tells the assembled firefighters that the fire is scythe action. The firefighter doesn't believe this, as Rowan clearly isn't a scythe, but Rowan kicks the firefighter to the ground and shouts his command again, which convinces the firefighter that he's a scythe. After the firefighter gives the order to fall back, he kisses Rowan's ring. Rowan feels dirty in Goddard's robes and thinks that he's become a monster.
Remember that impersonating a scythe is a gleanable offense; Rowan is taking a major risk by wearing Goddard's robes and acting like an ordained scythe. However, the fact that he's able to effectively convince the firefighter speaks to the fact that while evil, Goddard was able to teach Rowan how to behave in a compelling and terrifying way—something that's clearly useful.