Todd plunges his head in the river to escape the crowd of imaginary people looking at him, including a "boy" who is talking to him.
All the Aarons and Violas are gone, and the world is very bright. The boy looks like Todd, minus all the scars, and he asks Todd if they can manage alone; Todd tells him they don't have a choice.
He looks at the figures far away, trying to figure out if they're really Aaron and Viola. He can feel her silence, though, so he's sure it's them.
Fake Todd (a.k.a. the boy) asks Real Todd if this is the sacrifice that Aaron has mentioned in his Noise.
Fake Todd tells Todd that he can't kill Aaron, no matter how much he may deserve it. He also says that Todd's not a killer—he is.
Now we understand what's going on a little better: Innocent, pre-killing Todd is battling with his decision and his self that did kill.
Todd gets an idea: Instead of battling his alter ego, who is telling him that he won't succeed, he starts singing one of Ben's old songs.
He builds a fire and lights a long stick, giving it to Manchee, then they get into a rickety boat by the river. Todd's bad self is still taunting him, telling him that he'll fail, and that he can't leave this other self behind.
Todd frees the boat from shore and he and Manchee set off down the river.