Mind and The Stranger vs. Heart and The Slaughter
Mind is concerned with action instead of motivation or emotion. Heart is concerned with motivation or emotion rather than action.
The Stranger is concerned with 'people' that do things with no apparent motivation or emotion - people that act human, but they have no glint in their eye. Mannequins and wax models and taxidermy creatures can all act like the thing they represent, but they don't actually have souls. The Slaughter is concerned with emotion that has unknowable actions - soldiers that are motivated to kill but don't know when they will kill or when they will die. Anger that can attack at any moment.
Mind and The Stranger
Mind is concerned with masks/disguises, shaded glasses (covering up your eyes and thus soul), the role you play, and blending into the crowd. It is concerned with what the collective thinks is right to do. It is unbiased and emotionless. The Stranger is concerned with masks and disguises and blending in. Roles and masks are used within theatre and entertainment (which links to The Stranger's Circus), where actors say lines without necessarily feeling emotion behind those lines. They follow a script and wear someone else's identity - acting without feeling.
Heart and The Slaughter
Heart is concerned with instinct, emotions, passion, and your inner self. It is concerned with what you think is right to feel. The Slaughter is the fear of unpredictable violence, such as protecting yourself without thinking about your actions beforehand. Soldiers kill their enemies so they aren't killed themselves, and Slaughter-related characters like Melanie are very connected to their own autonomy and inner feelings, with her attacking Jon immediately after waking up from her impromptu surgery. She didn't have time to process what was happening, so she acted on impulse and emotion. Grifter's Bone compels people to murder simply by communicating incredibly strong emotion through music.
The Stranger and The Slaughter are opposites in this system due to the above ideas, as well as the fact that The Slaughter was the entity to stop The Stranger's attempt at the Unknowing in 1787, and their shared connection with music. You could also say that Tim's final action was an example of The Slaughter - he was ready to kill everyone in the wax museum just to stop The Stranger.
The Slaughter also seems to be a good weapon against The Stranger in general. The Slaughter attacks without caring who their victims are: they don't need to know who their victim is and thus don't care if that victim is human or not. Those that die on the battlefield are strangers to their killers, after all.
