'Horns' is apparently a horror-drama. In it, a man, Ig (played by Daniel Radcliffe with an accent I couldn't take seriously), has been isolated by the people in the local area after he is accused of murdering his girlfriend, Merrin (Juno Temple - she probably comes out of this the best). One day, he wakes up to find horns sprouting out of his head and that they have the power to distil the truth from people as well as telling them what to do. So Ig uses this power to try and find out who really killed Merrin. The answer is pretty much in your face, so there is no mystery or intrigue to be had.
Personally, I think the film should have had, at the very least, an element of fun to it. The way it is all done, comes across as very silly and doesn't even come close to resembling a 'horror'. And while there are times where it hints at a possibly entertaining film, it generally steers back to treating itself seriously (it would have needed a lot more subtlety to be able think about passing itself off like that) and ending up being a monotonous two hours.
Maybe it would have been interesting/engaging/entertaining had there been any actual horror or the central plot had something to give it some dramatic weight and keep you invested. But there is little use speculating. The film is what it is, which isn't good.