For most of the story, Martin seems like a pretty nice guy. Even Blomkvist believes him! Can you imagine that? It seems like he is a normal guy, with a calm smile and kind eyes. But while reading, we get to know a lot of awful details.
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. It is still unknown for readers if Martin was killing between 1966, the year Harriet disappears, and 1979, the year he builds his house. Can you imagine that it is more than ten years? Don’t be nicely left; this kind isn’t good at all.
Martin Vanger in the Essays