"I think that these crimes have something to do with the sex trade. And Salander would sooner die than be involved in something like that. I told you she's a damned moralist." (26.18)
It's true! It would totally violate her moral code and disqualify her from being a hero in a Stieg Larsson novel. That's why we're pretty sure it wasn't her that killed Dag and Mia right from the beginning.