Zalachenko is Salander's very bad dad, now in his seventies. He checks out early in the novel, just as we're getting to know him. Actually we know enough. He's an ex-Russian spy who defected to Sweden in the 1970s. In Sweden, he fathered Salander and her twin-sister Camilla. He never married Salander's mother, Agneta, but he returned periodically to have sex with her and beat her. After he beat her unconscious (leading to her premature death), Salander (at age twelve) threw a gasoline bomb into his car and followed with a match, nearly killing him in the fire. This act is used as evidence to support claims that Salander is mentally ill, and remains a topic of debate for readers and characters.
In any case, things get pretty tense early in the novel when Salander and Zalachenko are recovering two rooms down from each other in the hospital from wounds they inflicted on each other. Each is bent on finishing the job he or she started in The Girl Who Played With Fire – murder. Though Salander would sure like to kill her dad, she doesn't get a chance to. A member of the Section, Gullberg, ends up fatally shooting Zalachenko. His death certainly makes the plot more manageable and it gives readers a wake-up jolt. It also foreshadows Salander not killing her brother, Ronald Niedermann.
For more on Zalachenko, be sure to check out our thoughts on him in The Girl Who Played With Fire.