I watch American Beauty once or twice and year and every time I see it, I wonder what the purpose of setting up the ending in this way is. Why have so many of the characters been placed to be suspected of the ending murder? How does that serve the themes of the film?
The opening line is "My name is Lester Burnham. This is my neighborhood. This is my street. This is my life. I am 42 years old. In less than a year, I will be dead."
This line sets up the mystery of the film. The murder and immediate reveal of the murderer at the end resolves the mystery - it's a who's going to do it instead of a who did it. Presumably, I know this is fiction, but if the story after the murder was worth telling it would've been told in a sequel or an extra 30 min coda but the whole crux of the film is how and why Burnham will die so when this is resolved the story ends - we see that in spite of their troubles during the film the emotional reaction from Burnham's family and how, deep down, he was loved - this leaves the viewer with a more positive feeling and any further detail would detract from this.