The seeker is already a player-controlled agent, but the idea of having hostile agents turn insane due to what they learn is a good one. Maybe the seeker could choose to leave some of the clues they find behind, so any investigator who investigates the clue slowly goes insane. The agents already have a sanity value, because they have a full person, but currently that sanity value does nothing. With just a bit of changes (and the UI has space for it), this could be a different way to defeat your enemies, or sow confusion inside their ranks.
I'll see if it can fit into the next version.