I still don't get why Nick thinks embracing the degenerate choice would change the outcome of the movie, he would still be shot dead. Symbolically he'd be fully free from societal norms or some stupid shit by fucking a teenager? Lester would just be fully immoral and there would be no point to his character or the movie other than Lester being a miserable piece of shit who fully deserves to get his brains blown out.
So why was Lester shot dead?
- Ricky and Lester's daughter talk about how she wants her father dead because she knows he is creeping on her 16 year old friend. She says it was a joke at the end of the conversation. Not that much later "Ricky" told his dad that Lester paid for sex with him. After telling his dad that, Ricky announces to Lester's daughter that she should run away with him to live in New York that night.
Ricky and his daughter come in after Lester is shot. The script says this:
Ricky comes into the kitchen and slowly approaches Lester's
lifeless body, wide-eyed but not afraid. Jane follows him, in
shock. Ricky kneels, gazing at Lester's unseen face... then
he smiles, ever so slightly.
The implication of Ricky smiling is that he knew exactly what he was doing. That he indirectly caused Lester's murder and is now going to walk away with Lester's daughter.
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I've got one theory about how Nick could believe that Lester could have been "saved" by having sex with the 16 year girl.
The idea would be that if Ricky's father had seen Lester having sex with a 16 year old when he came to the house to kill Lester, he would realized that Lester was straight and would not have killed him.
I don't think that really works in terms of the film's plot, but I can see that as being something Nick would attach himself to.
I think its possible to make a more convincing libertine argument that what killed Lester was his refusal to embrace ultimate "freedom" and to have sex with a man.
None of this really matters. As has been said before, the film is an ink blot test where people can find any meaning they choose to find in it.