Frailty/Fridge

Fridge Brilliance

When Fenton is initially told about the first vision his father had, he instantly goes through a Heroic BSOD. Both Adam and the father initially believe that he is having a hard time taking in the new information and we the audience initially believe that he's realizing his dad has gone nuts and will kill people. When it turns out everything is true and Fenton is also a demon, it could be interpreted as the Demon within Fenton worried he'll be found out.
At the end, Adam was revealed to be the new sheriff of his home town. How was he able to get high of a rank? Since he had his sin seeing power that his father has, he could use it to tell which arrested crooks were innocent and guilty.
Adam's secretary is shown to be pregnant in the end of the movie and has her hands on Adam in a caring manner during the final shot. It seems Adam plans to start his own family of demon slayers.
When the Father makes his second kill, he specifically mentions to Fenton that the man he destroyed, was a murderer of children. He was later proven right in the end. That was the only time he mentioned the sin he claimed to have seen. Because Fenton supposedly couldn't see the first "Demon's" he thought maybe telling him what he did would help him see it better. This becomes the first clue of Fenton becoming a demon.
A double whammy of one in the beginning of the movie. When Doyle calls the Sheriff's office to confirm Fenton's/Adam's story, the Secretary says that the Sheriff is out for the night. That's because The Sheriff is sitting in the office with Doyle. The other whammy is more of an easily missed moment. When the Secretary answers the phone, she answers with "Meik's Sheriff's office." Not only should that have clued Doyle in, to "Fenton's" identity but it also proves that God can interfere with the perception of others to protect the Meiks and demons aren't immune to it.
When Fenton tries to get the Sheriff to the shed where most of the killings took place, the Sheriff, while in disbelief; asks the Dad to check it to quiet Fenton down. The Dad asks in a cryptic fashion "Well if it has to be done, then it has to be done. What do you think Kiddo, does it have to be done?" initially you'd assume, he was referring to checking the shed. However it later turns out that the Dad, was asking Fenton whether or not he should kill the sheriff. The sheriff was dead the second Fenton asked him to check the shed.

Fridge Horror

When Adam mentions that he couldn't kill Fenton until he was on the list or it would have been murder, that would mean that up until Fenton became the God's Hand Killer, Fenton was still human. Killing his father wouldn't have been enough to make him a full fledged demon. Meaning God only put Fenton on his Father's list as a test of faith and not really as a demon. Hence why the father didn't want to kill Fenton, not just because he's his son, but he was still human and he'd react the same way as he did murdering the sheriff.

Fridge Logic