Okay, so from what I've read elsewhere, the explanation of the ending seems to be that: Maura suspected Billy in the recent murders around town. She loved him though, and wanted to "save" him. So the two move away together to the highlands. One day, a neighbor woman asks Maura if she's seen her missing dog. When Maura heads back to her cottage, she hears Billy playing the harmonica, which (for some reason) signals to her that Billy will go out tonight and murder the woman. Maura heads over to the cliff, and blocks the path to the neighbor's house. Soon after, Billy shows up on his motorcycle, thus indicating that he was heading over to the woman's house to murder her. He sees Maura's tears, knows that she knows about him, and so he commits suicide by driving off the cliff.

Here's what I still don't understand.... The harmonica. Why would Billy playing the harmonica signal to Maura that he's going to commit another crime? Prior to the ending, I only recall one other instance in which he played the harmonica, and that was shortly before he and Maura had sex.

Also, was Billy telling the truth when he said that he knew Jeffrey (the old guy's nephew) and heard about the gardening job through him? Was Billy telling the truth about the farm house burning down and his mom dying? Was he actually Mrs. Prince's great nephew?

When Billy was telling Maura and Mrs. Prince about himself, I got the impression that he was lying about everything (he had no references, lied about church). But was most of what he'd said actually true?