Not much actually happens in this movie. There are a few pivotal moments, and everything else is talking about a crucial moment that takes place before the movie began. The primary mechanism used in the story is the flashback. Flashbacks can be used very well, but they aren't here. There is zero indication when flashing back or forward, and there are only weeks separating the events, so context as a scene unfolds is the only way to know when something happens. Perhaps this was intended to add a sense of mystery in places, but it was largely annoying.
If you are interested in watching a 100 minute on-screen discussion of the why's of things and a lame questioning of good versus evil, enjoy. If watching close-ups of a mostly vacant stare on Ryan Gosling's face for much of the movie appeals to you, you're in luck. Sadly, I can find no reason to recommend this movie. Oh, and as another review indicated, Spacey really is only in a cameo role here, and plays with a disinterested detachment that you've probably already seen.
*** The rest contains spoilers though a few big points are held back ***
First, the killing of the so-called retarded boy happens before the movie opens, and we never learn what happened exactly. The implication is that he was killed to stop the deep sadness that was communicated through the boy's eyes. There aren't many other unanswered questions as the characters beyond the boy and the lead are very one-dimensional.
As for what is with the lead character... Another review suggested he is crazy. I agree he isn't 'right', but don't know that I'd call it crazy. It seems that the movie tried hard to convey that he was either autistic or retarded in some way, because he sure seemed slow and unable to grasp the obvious. Either that or they were going for a neglected youth with some detachment disorder and/or who was heavily medicated. As always Ryan Gosling is good at conveying something to the audience, but it seems exactly what was deliberately left up in the air in this case. This movie provides no answers, just questions, but it was a sufficiently bad movie that I really didn't care.