The Fentons' used everyone's distraction to take Emily and you as hostages. "Turn back our son into a human and disappear with him, or we kill those two! We're not kidding!"
Mr. McAdams was outraged by how the Fentons acted. While he didn't hesitate to defy the First when he thought they were about to kill other gods, he was able to understand how misguided he had been in thinking beast-peoples were the source of Serra inability to produce new souls and shouldn't have one.
Sure the First could make wrong decisions, the god himself had admitted as much. But said entity was already around when Serra became barre and, if they says humans are the cause, is more than inclined to believe it. In the light of this new knowledge, he saw how his past beliefs caused him to do wrong to a point it warranted punishing him. And for what he knew, the punishment given the Fentons' son was rather lenient.
So why? Why couldn't his parent accept this like he did, now that he have all the facts?
Noticing that the two demented parent looked at everyone but him, he slowly placed him behind them. Then, he looked at the two gods, nodding at them, before acting like he had a dagger in each hands and wanted to plunge them in the necks of your kidnappers.
Understanding what Mr. McAdams tried to do, the gods turned the two imaginary daggers into reals ones mid-way. They plunged into their flesh, their bodies slackening, letting Emily and you free. The two of you ran to your mother, wile the Fentons parents fell to the ground, bleeding to death.