Peter was dragging Lucas under when he heard someone calling to him.
"I'm sorry, Peter. He's just a little boy. Take me instead."
That was Jake. He used to follow Jake and Bill around. He had idolized them. They had pretended to be his friends and then drowned him. He'd been angry ever since.
But, as he heard Jake's pleadings, he realized that what he was doing was wrong. He was no better than Jake or Bill. He was killing their innocent offspring.
He let go of Lucas. He wasn't doing it for Jake. He was doing it for his mother.