Like many titles of paperback originals, this one has no connection to what happens in the novel, at least none that I can detect.
The hero of this short novel is Kid Collins, an ex-boxer, so punch-drunk he flew into a rage and killed a man in the ring. Thompson wrote other books featuring killers, notably The Killer Inside Me, but the hero of After Dark My Sweet, is different. He knows killing is wrong and knows he can't control himself.
Collins runs into Uncle Bud, a grifter who has a plan to make a quick fortune. Collins joins in the plan, but Uncle Bud apparently doesn't know how dangerous he is. When the day comes to put the plan in motion, will Collins do what he knows is right, or will he lose control?
In the film adaptation of the same title, Jason Patric gives a fine performance as Kid Collins, and Bruce Dern as Uncle Bud once again shows the world how to play the kind of characters that reliably turn up in noir films.
Rachel Ward gives a fine performance as Uncle Bud's girlfriend, Faye Anderson, but readers of the novel won't recognize the character. In the novel she's an alcoholic in a downward spiral, and her lust for Kid Collins has Oedipal overtones. The screenwriters made her into a woman who is too young for Uncle Bud, and therefore a better match for Collins.
It is not unusual for Hollywood to make noir a little less bleak.