Who kills Lester Burnham in American Beauty?

Who kills Lester Burnham in American Beauty?

***Movie Spoiler***

Answer: 

Col. Frank Fitts, USMC murders Lester in the movie ending. Why? Fitts is embarrassed that he kissed Lester. He assumed Lester is gay since his son is always at his home. He once watched Lester laying on his back, which he mistakingly thinks his son is giving oral pleasure. 

However, Ricky is selling high quality marijuana to Lester, who is living his life to the fullest after accepting to leave his job and that his wife is having an affair. 

Fitts kisses Lester in the garage. Lester tells Fitts he is not that way. The retired Colonel returns later to shoot Lester dead in order to protect his gay secret. 

The end