The primary difference between what qualifies as serial murders and what qualifies as mass murders is the selection of victims. premeditation. a cooling-off period. the number of victims.

I believe the appropriate answer is a cooling-off period. The cooling off period us described as the state of returning to the offender's usual way of life between homicides. The key distinction between the serial killers and mass or spree killers is this emotional cooling off period in which the killer blends back into his/her normal life.