My very personal interpretation of the film (Spoilers)
My interpretation of the film is that "The Guy" plays Satan. However, he is not the traditional Christian Devil, he is more like a Nietzschean Devil: God is dead and his values no longer exist on earth. Although, Satan has no power over the world.
The word Satan means "the opposer", but if there is no God in the world then Evil has no one to oppose, whereby Satan roams the world yearning for the good. Satan's nostalgia leads him even to do good, which actually executes for The Girl. She is pure and somehow she inspires him to do good in his own way: killing her stepfather, beating the guard, forcing her to walk on the pond. That's why he can't sleep with her but he does with the other girl, because the traveling woman is not pure. This creature can distinguish between good and evil and even can expel it.
The way he walks down the field and the way he looks at the sky, shows him expectant but for what... for God. He shows his melancholy, the paradox here is that he once wanted to be greater than God, but without a God, Satan becomes an actant of good. In the past Satan was an angel, right? It's as if the Devil would have been sick of the non-existence of God and would have done good because evil is no use if the power of God is gone from Earth.
Well, who knows. I find this movie quite rich in interpretations and liked it.