Saving everyone is probably the ideal situation for The Devil in Me but there’s an option to kill everyone and get the solo story achievement.
This guide is for the original versions of the game and not the curator’s cut. This is meant to highlight the most optimal way to get everyone killed.
Erin – The Room
When inside the room alone with the killer, attack him to get Erin killed. Another option is the breathless chamber, choose Erin to die.
When inside the fire trap choose the door option twice.
Before the wall chamber trap, make sure that Kate has the screwdriver. Or get Kate killed by choosing Jamie to take the screwdriver and hitting the button. Another option is to ask Kate to help her when with Connie the Dog, choosing to spar Connie while she barks will alert the killer and murder Jamie.
Kate will evade death until the very end unless the player kills her in the wall trap. Even when saving Connie the killer will miss on purpose to avoid killing her.
Mark will survive until the end and will be placed in the chamber. I tested this by doing nothing during QTEs and saying nothing throughout the campaign.
To get the Solo Story achievement to make sure to take the killer’s deal. This will restart the cycle ending with the achievement unlocked.