The chief protagonist in the story, set in Capetown, is Michael Adonis, a young black or Colored (the South African term for one of mixed race) man who has just been fired from his job. Michael understandably seethes with resentment against the white people who mistreat him: the manager at work who has him sacked simply for taking restroom breaks, and the Boer policemen who stop him on the street for no reason, questioning him about where he got the money he has and demanding to know if he's holding drugs, when all he is doing is minding his own business. In a discussion with friends, Michael is told about a lynching in the US that has been reported in the paper. One man mentions that the capitalist system is responsible for the racism that is endemic in both South Africa and America.
The effect of all this is to create at least an undercurrent of a desire for revenge within Michael. But there is no outlet for it; the perpetrators seem unassailable. When Michael visits the neighboring flat of a white man named Doughty, an alcoholic, has-been actor, he strikes out at Doughty with a bottle, accidentally killing him.
Though Doughty has apparently been trying to be nice to Michael in his drunken way, Michael has misinterpreted it and probably sees him as symbolic of the entire establishment and its mistreatment of non-white people. He's predisposed against even a harmless and pathetic man like Doughty, and the man becomes a victim of Michael's displaced revenge against white people. Though his killing of Doughty was unintended, Michael probably realizes that in some sense it was not totally accidental, and given the dynamic of the apartheid society, it's easy to see why violent incidents like this would take place.
From the opposite side of the conflict we later see that the police constable Raalt is a man with grievances of his own, but of a different nature because his own wife is the object of them. His personal domestic problems thus feed his own resentment and aggression, causing him to take out the anger on convenient victims—especially, of course, black people. Raalt targets a man named Willieboy, a friend of Michael's, for the killing of Doughty and shoots him, and Willieboy dies in the police van when Raalt refuses to call an ambulance and deliberately delays getting Willieboy to the station. It's as if this too is a twisted and displaced form of revenge against one or more people whom the aggressive and abusive Raalt holds responsible for his own problems.
The entire white establishment can be seen carrying out a form of displaced anger, though "revenge," even in an indirect form, is not in general the central drive animating it. Obviously the reasons for racism are complex and multi-faceted. "A Walk in the Night" is a study in human cruelty, of which revenge, whether direct or displaced, is only one manifestation out of many.