Liked it but it made no sense
I enjoyed this movie, I'll start there. The characters were interesting, the mystery unfolded well, and the acting was good (despite a few rough American accents - actually Radcliffe's was excellent, I thought). I even liked the "uneven" tone a number of people are complaining about. The humor stopped as soon as the really serious ramifications were spelled out (about half-way through) and I thought it actually served to heighten the suspense. That was something Hitchcock did a lot, and it seems to work here.
OK, SO -- what I didn't get was - why the horns? Why the devil transformation? It didn't make sense and it was never explained or even hinted at. Does the devil hate murderers and help god punish them? That would be an interesting take if slightly touched on, but it wasn't. Iggy wasn't guilty at all so it wasn't his guilt manifesting physically, which is a tried and true literary device. It was random.
And why would the victim's cross protect her murderer? That makes no sense from a storytelling standpoint. Magical/fantasy plots like this only really resonate if there's some sense of balance, symbolism, or poetic justice.
The sort of genre that DOES invite random punishment for both the evil and the good is Horror. You can get away with this sort of thing there, but I'll propose that this was NOT a horror movie in any way, despite a few trappings. It was a mystery/romance with a touch of the crime thriller, that threw in some slightly disturbing Fantasy elements. But Horror, it was not.