Were you totally surprised to learn that Niedermann is Salander's half-brother? We were, and so was Salander! Niedermann is a pretty outrageous character. As Paolo Roberto discovers, Niedermann has a bizarre medical condition called "congenital analgesia" (30.55), which leaves him unable to feel pain. Plus, he has superhuman physical strength and particularly hard limbs, lethal weapons unto themselves. As Salander points out, Zala has trained his son to be a killer. However, Niedermann is also prone to hallucinations and is afraid of the dark.
When we see Zala and Niedermann together, we see how very sick the relationship is. Zala doesn't care anymore about his son than his daughter. He keeps Niedermann around because he can use him to do his dirty work. We could almost feel sorry for Niedermann.
But, Salander would never forgive us. This is the man who was ready to go Texas Chainsaw Massacre on Mimmi, after all! Think he was bluffing? Tell it to the dismembered bodies found around the warehouse he burns down after Mimmi and Paolo Roberto escape from it. He's also been beating up and killing victims of the sex trade, and, of course, murdering Dag Svensson and Mia Johansson, who Salander is accused of killing. He kills Nils Bjurman as well, but we can't get very worked up about that. It is ironic though, that Salander's brother and "diametrical opposite" (31.113) is the one to permanently rid her of that problem.