Why Not Kill Stuart?
Brian was about to kill him but the daughter stopped him. Brian then explained to Stuart that he'll just get a good lawyer and be let out of jail in a few years.
Obviously it is much worse than that. Whether in jail or not, Stuart could hire someone to kill the daughter any time he wants. And, Stuart may not even go to jail. As far as I can see, there is no evidence that he killed Lenore and all the witnesses are dead. A good lawyer could easily get him off the hook completely.
But, if Brian just killed him on the spot, and called it self-defense, he'd never have to worry about his daughter's safety ever again.
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What Would Jesus Do For A Klondike Bar (WWJDFAKB)?