Frailty is absolutely fantastic.
Matthew McConaughey goes to the FBI to tell him he knows the identity of the G-d's Hand Killer: Adam Meiks. To make the FBI agent understand, he begins to tell him a story about when he was younger and his father used an axe called OTIS to "destroy demons" in front of him and his brother. Fenton thought what his dad was doing was wrong, but Adam knew it was right. As a result, the dad put Fenton in a hole and had him pray to Jesus until he knew the truth. After about a week, Fenton comes out and declares he knows the truth now.
Fenton kills his dad because he must have been a demon too. Then Fenton grew up and became a serial killer. His brother, Adam the G-d's Hand Killer, destroys Fenton. And then goes to see the FBI agent in charge of his case.
This movie is amazing. It's very, very well directed. (Mad props to Bill Paxton.) It's an old school Gothic horror film. Frailty definitely creeps into your head and lives there.
The audience is never quite certain whether Dad Meiks is completely insane or actually doing the work of G-d. The ending
which shows Adam seeing all of the FBI agent's sins before he kills him makes this really ambiguous. It could be an unreliable narrator or it could be truth. I think that works really well.
Also, the kids chosen to play Fenton and Adam are amazing. They're excellent actors. Adam can be annoying at times, but he's not a perfect child anymore than Fenton is. The kid who plays Fenton is really amazing. The only other thing I've seen him in is the also fabulous Brick starring Joseph Gordon-Levitt.
I highly, highly recommend this film.