In the game, Space Marine lead character captain Titus is able to resist the warp energies. How was he able to do this? Is he corrupted or will he turn traitor? Any stories about what happened to him after the events of the game?
Captain Titus is a blank, he is invisible in the Warp (only in story, in-game bloodthirsters can see him) and immune to its effect, as @Bosc said. The Space Marine was to be a trilogy of redemption(after the first game of course).
The second part of his story was to focus on a ‘Titus Unleashed’ plot—basically there were forces arrayed against him that would see his loyalty to the Adeptus Astartes pushed to its limit, and his reaction would be to kind of ‘go rogue,’ and we’d see a different Titus, not quite as in control as we saw him in Space Marine. He would be kicked out as a consequence—exiled, which would basically be a death sentence for him.
He would survive, and come back even stronger in the third game, where other Space Marines still loyal to him would rally around him and he’d return to ‘clean house,’ but as the head of a brand new Chapter that we would build around him.
The story comes from this interview with game director Raphael van Lierop.