Spoilers below
Dad sees an entity claiming to be an angel. This could be a true visitation from beyond or a hallucination. We have no way of knowing for sure, although events near the end seem to indicate that some form of supernatural intervention has occurred.
Assuming that there was some supernatual visitor helping Dad and Adam, is this visitor good or evil? It claims to be an angel, but suppose it was a representative of the other side coming in the guise of an agent of good. Dad and Adam see visions, but we don’t know if the visions are true.
The angel gives Dad and Adam lists of “demons” to destroy, and the power to see the demons’ “true form” by touching them. Both Dad and Adam emphasize that they are not killing people, they are destroying demons that are disguised as people. Just before killing them, they touch the people and their evil deeds are revealed. But, even assuming that the visions are true, what Dad and Adam see are not visions of demons, but of people doing evil things. Regardless of what they say and believe, they are killing people; evil people perhaps (though we can’t know for sure), but people nonetheless. Since the “angel” lied to them about this aspect of what they are doing, we have no reason to believe that the visions it is sending them of the people’s actions are also true.
Also, they don’t recieve the visions of the people’s evil deeds until the last moment, which is after they’ve already decided to kill the people. This sounds awfully like a self-fulfilling prophecy, ie, only after deciding that these people are actually demons that must be destroyed do they recieve a vision of their victims doing evil things.
Then there is the problem of the Sheriff, Fenton, and the FBI man. Dad kills the Sheriff, an innocent man, solely because he knows what Dad has been doing. Adam kills Fenton because Fenton poses a threat to Adam’s activities. The FBI agent likewise poses a threat to Adam’s killing, and though Adam does get a vision near the end, as I said earlier, we have no way of knowing if this is a true vision. And even if this vision is true, it seems a strange coincidence that of all of the people in the world who have done evil things, the one marked for execution is one that poses a potential threat to the killing spree. Here we have at least one person, and possibly as many as three, being killed solely because they are a threat to Dad/Adam, and thus are the enemy. If a good man (the Sheriff) can be the enemy, then the assumption that all of the enemies of the angel that are marked for destruction are evil no longer holds up.
Think about it this way. Suppose a supernatural entity claiming to be an angel and agent of God came to you in a vision with a list of its enemies to destroy. How could you tell if the entity was an agent of good telling the truth, or an agent of evil telling you lies? How could you know for sure?
I think that in Frailty, the evidence isn’t conclusive either way, but given that at least one innocent that we know of, the Sheriff, was victimized by the Hands of God, and that the angel seems to have lied about at least one thing, it seems stronger towards the latter possibility.