Martin is CEO of the Vanger Corporation, and one of the novel's chief villains. For most of the story, he seems like a pretty nice guy. Even Blomkvist is taken in. Plus, he's on the other side of the bridge when Harriet disappears, so Blomkvist doesn't really see him as a suspect in her disappearance. Of course, that's because although Harriet's disappearance is linked to Martin, he actually had nothing to do with it and is as puzzled by it as everyone else.
Martin didn't kill Harriet, but he does kill lots of other women. As a teen he witnesses his father's rapes/murders, and participates in at least one. He kills a woman when he's away at school, the year that Harriet disappears. We don't know for sure if Martin was killing between 1966, the year Harriet disappears, and 1979, the approximate year he builds his house on Hedeby Island. That's over ten years. But we can bet he was up to no good.
From Salander's perusal of his "death book" we know that from the time he built his house in the late 1970s he kidnaps dozens of women, tortures them in his basement, and then kills them. He tells Blomkvist,
"[…] I'm more of a serial rapist than a serial murderer. […] most of all, I'm a serial kidnapper. The killing is a natural consequence – so to speak – because I have to hide my crime." (24.26)
After he's done with the women, he dumps their bodies at sea with his speed boat. Like his father before him, he has all the money and power he needs to commit these crimes without being noticed. Yet, he's more cunning that his dad was. He chooses the most vulnerable women – prostitutes, immigrant women, women with drug problems. He chooses women whose families, if they have families, won't notice they're gone, or won't have the means to pursue a major investigation. We don't know exactly how he gets the women from the kidnap sites to Hedeby Island. But he's rich enough, smart enough, and lives in an isolated enough area, to think of plenty of ways.
Martin presents himself as something of a foil to Salander and Blomkvist. He tells Blomkvist, "Of course my actions aren't socially acceptable, but my crime is first and foremost a crime against the conventions of society" (24.25).
Blomkvist and Salander both live outside of the conventions of society in many ways. We think they're clearly out to help people, not harm them – as long you aren't a bad guy. But we believe this because we share some of their values. And most readers will take issue with at least some of the things they do – Salander in particular.
Martin doesn't even seem to think about his victims as beings who have value. The women he hurts haven't done anything to him. This obviously has little to do with "conventions of society" and more with his lack of value for human life. This, as Blomkvist suggests, might be because his father treated Martin like a thing, and so Martin can't really see the value of human lives. Salander suggests that it's not that complicated. Martin hates women and gets genuine pleasure from hurting them. "Why" doesn't matter to her. All that matters is stopping the guy, and providing justice for his victims.
In any case, Salander and Blomkvist foil Martin's plans. After Salander beats him up with his own golf club, he manages to drive off (with one arm hanging on by a thread). But with Salander in hot pursuit on her Kawasaki, the semi truck coming at him head-on looks pretty good. If he hadn't taken his own life, would Salander have killed him? Or, like she tells Frode, would she have tortured him and tossed him over to the authorities? We know she doesn't like authorities, but she does decide matters on a case-by-case basis. What do you think?