This was one of my favorite films in a long time. There isn't a second of screen time wasted. That kind of economy is so important in film and so few films have it. Also, in other similarly plotted films, I find myself hating the character who makes up the lie, but Vinterberg made it so the audience understood exactly why she lied and captured her guilt perfectly. This is a film where you feel bad for pretty much every single character.
I think the last scene is supposed to be ambiguous. It doesn't matter who shot at him. The point is that Lucas will be haunted by this for the rest of his life.
I wonder what film you are talking about? This is a story I have written, not a movie.