I don't see any "devil" in the ensemble cast plot. Why is the movie titled "The Devil all the Time"?
With the continuing occupation of Afghanistan overtaking Vietnam as the longest war in American history, The Devil All The Time's unsparing critique couldn't come at a better time. As the film's title tells audiences, Arvin's father spent his whole life fighting because the enemy killing him was at home in America all along.
The Devil All the Time stays true to its title in its embrace of constant darkness and shows viewers only the worst that humankind has to offer — meaning audiences hoping for a glimpse of humanity or a purpose to the madness are shit out of luck.
While the presence of God is a nebulous thing in this backyard fried noir, ‘the Devil’ of the title is present to just about every character inhabiting Pollock and Campos’ vision of Knockemstiff, Ohio and its surrounding areas: It is the hell they make for themselves and their heirs by pretending to be better than they are while ignoring the pain underneath.
Early in the movie, the narrator says:
Years ago, Willard had fitted together a weathered cross above a fallen tree in a small clearing behind his house. He came every morning and evening to talk to God. It seemed to his son that his father fought the Devil all the time.
This means, that the fight between good and evil is constant in the lives of the characters in the movie.
There can be also another explanation: Notice, how many people in the movie make some bad (or plainly evil) things based on the "voice of God": Roy kills his wife, believing that he can resurrect her; Carl kills his victims because "he can see God in their eyes", girls seduced by Reverend Preston believe that they are getting closer to God. But it wasn't God, but the titular "devil all the time" who was giving them advice.