What is the name of the doomsday prepper killer in season 6?

The name of the Doomsday prepper killer in Dexter season 6 is Travis Marshall.

Travis is introduced as a Bible-restoration researcher who becomes the central figure in Miami’s Doomsday Killings. He works under Professor James Gellar, whose religious teachings persuade Travis that the murders are part of a divinely mandated apocalypse.

A major twist reveals that Gellar has already died and that Travis has been imagining him as a living mentor. Travis nevertheless continues the elaborate biblical tableaux, including plans involving Wormwood and a deadly gas attack. His actions make him the season’s main antagonist, while his guilt and psychological instability give the storyline a tragic dimension.

Travis is sometimes confused with Professor Gellar because Gellar appears to direct the killings for much of the season. Gellar is the ideological influence, but Travis is the person carrying out the murders. Dexter ultimately stops Travis in the season 6 finale, “This Is the Way the World Ends.”