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I ain't never killed no one before that, Will.
— The Schofield Kid · Unforgiven

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker admits, to someone called Will, that they had never killed anyone before that point. The plain, rough phrasing gives the confession a sober weight, treating the act as a line crossed. The double negatives sound colloquial, making the admission feel unpolished and raw.
Context reviewed