Why am I so surprised when two of my PCs turn out to be evil?

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Post-game, the chatter was mostly focused on game mechanics.  The two PCs behind the massacre shrugged that move off as something their characters would do.  And I mean that literally.... shrug... "I was just playing my character."  The other player's response was "if there is no law or obvious repercussion, I will generally default to the most violent (i.e. quickest) option to resolve a given situation." 

Of course there's a law, it's just not the police and laws we think of generally. But hey, the law of payback's a worse bitch. And here's where you make those dead NPCs as human as the player's own mother: tie survivors to the PCs as family, old lovers who parted on decent terms, childhood friends who don't understand why they turned on the crowd. Now is a great time for a threat move that's all about the PCs humanity. Check over the Threat Moves and look for ones that hit from a totally different and non-violent angle - revealing things, seeking help or comfort, asking for help, putting the consequences in their path everywhere, etc, etc. Heck, use the gift of the World's Physic Maelstrom to plague them with ghosts. What does the visionary girl want now?

Yeah! I'm with Margolette. If the players are still invested CoR, then you have a great game on your hands that has had the tone set by the massacre and yet can have nothing to do with killin' shit and taking their stuff.

I know! Make the dramatis personae's deepest thoughts on the massacre a MC stake. Its a question you want answered in play. Tie your hard moves back to the massacre. Remind the players of it through your moves. It may not have been a big deal to the players, but 'address the characters, not the players' , ask probing questions that strive toward your rattled state of mind.

Although you look at your NPCs through crosshairs, make them the player's choices. Make them costly choices too. Not just to their goals and aspirations, but to their humanity. Nobody likes a psychopath or wants to deal with an anti-social killer. Revenge or Banishment is a very real possibility.