Why does Colonel Fitts kill Lester at the end of American Beauty?

This confused the hell out of me. I thought it was his wife who kills him at first.Why does Colonel Fitts kill Lester at the end of American Beauty?
fitts is a strict disciplinarian, to the point of emotionally traumatising his wife and making his son take regular urine tests. he is clearly emotionally and sexually repressed, and is probably a homosexual. this goes against his being a former army man who is supposed to be manly and tough. so he sees his son with lester, and that time when he thinks his son is going down on lester makes him ashamed that he produced a gay son, but also he feels ashamed that he himself is probably gay. he admits as much when he sees lester working out and appears to be attracted to him. when lester rejects him, it is the ultimate humiliation, and so he punishes him for his own shame by killing himWhy does Colonel Fitts kill Lester at the end of American Beauty?
I think it's a combination of Fitts thinking Lester is gay and Lester rejecting fitts when he goes to kiss him but the main reason is cause Fitts has some deep psychological problems
because he thinks that he is paying his son for sexual favors. Remember when he was watching his son roll Lester a joint? It kind of looked like he was going down on good ol Lester....