The Dad's death was the angel's punishment for not killing Fenton.
The dad couldn't bring himself to kill his own son demon or not. So he decided to leave it to fate on what should be done. During a routine Demon hunt, Fenton instead killed the Dad. The whisper he gave to Adam was warning him not to make the same mistake.- I thought that Dad was killed as punishment for killing the sheriff. Masquerade or no, it was still cold-blooded murder.
When Adam shook the FBI agent's hand at the end he passed the God Mission on to him.
Why else would he told him he was a good man when most people he and his father touched were Demons? Any other innocent people they put their hands on had no effect whatsoever. It was the Devil who gave them the mission, not God.
As stated in the Artistic License – Religion entry on the main page, demons are immortal spirits, and killing possessed people only kills the person not the demon. Taking that into account, even with the supernatural elements, the Meiks' killings don't really do anything besides kill people (though, admittedly bad people). Doesn't tricking a family into killing in God's name seem like something Satan would do? And as a bonus, all the evil people the Meiks kill go to Hell early too.