Not sure where Dexter has set up this particular death tableau, but he's managed to fit in photographs of all Arthur's victims, plus that damn toy train. When Arthur comes to, he seems less surprised than you'd think that Dexter is doing this to him. After Dex slices his cheek to get his blood sample, Arthur sneers at him, saying he never took this much pride in the crimes he committed. That's awfully uppity for a guy who bashed poor Sandwich Guy's brains in with a hammer. But Dexter's more concerned with punishing Arthur for his crimes against his family. Arthur thinks he gave his family everything, but Dexter would beg to differ. As would Christine who, Dex informs Arthur, killed herself. Arthur reacts to this with both sorrow and anger ("She was weak!"). Dex says he can't believe there was a time he thought he could learn something from Arthur. I know right? I can't believe there was such a lengthy period of time between that time and this moment here, where Dex finally gets to killin'. "You think you're better than I am?" Arthur spits. "No," says Dexter. "But I want to be." Arthur is dubious that Dexter can control the demon inside of him, any more then he could. Dex asks if he ever tried, and Arthur says he did; prayed to be changed. "That's not trying," Dexter observes, "that's just waiting to be stopped." Arthur thinks it worked, given current circumstances.
Dexter now seems like he wants to talk, get some advice going forward. He asks Arthur what the alternative is: leave, disappear, fake his own death? Arthur says those won't work -- he'll still be who he is. "I have a family too, Arthur," Dexter says. Arthur doesn't say a word but raises an eyebrow like you wouldn't believe. Granted, that's the only body part that isn't currently bound in thick plastic, but still. "I'm good for them," Dexter tells him. Arthur then starts yammering about heaven and God and fate, and Dexter has no patience for that. "It's already over," Arthur says, then repeats it. He turns his eyes to his train set and asks Dexter for one last kindness. Dex turns the train set on, and puts on one of Arthur's old records. And then Dexter dons his face shield, grabs a framing hammer, turns it claw-side-out, and ends the Trinity Killer forever.