Board Thread:General Discussion/@comment-5413572-20160521065337/@comment-27907368-20160527035929

Not quite, because we see that they are upside down even outside of the battle room, during the barrier break scene.

I believe that it may be symbolic; in the Genocide Route, the protagonist's damage gets very large (999999) near the end. The monsters' SOULs could be a reflection of our own - if we want only to kill, their SOULs reflect that in damage taken. If we hesitate, if we befriend all monsters, we deal little damage (such as in the Undyne date - we deal "1").

The monsters hearts and the damage they take is a reflection of our character.