the difference between a mass murderer and a serial killer...sociopath vs. psychopath...Oh the fun we can have.
These are my thoughts/definitions:
mass murderer: One who kills a large number of people (more than five??) in the same episode. I think Yates fits the bill here.
serial killer: One who kills one person at a time over a period of years, with sexual violation and/or torture involved. The brutality usually escalates over time.
sociopath/psychopath: same thing in the psychiatric literature, I believe.