The singular motive isn't always clear-cut, especially since sociopaths tend to be
both con men and sadists (though you could argue that a con man who enjoys cheating (and therefore negating) people is a sadist already). Take the case of my personal favorite among serial killers,
H.H. Holmes: he was clearly a sadist who enjoyed exerting power, but he seems to have killed for financial gain, to avoid discovery, and after he'd tired of willing female playthings. According to
The Scarlet Mansion (which Amazon mistakenly calls "lightly fictionalized" -- they mean lightly dramatized), he's said to have committed his first murder as a child. I have yet to hear anyone call Holmes something other than a serial killer. Read the descriptions of the murderous machinery in his house.
Remember that Robert Ressler,
the man who coined the term serial killer, intended purely to distinguish between numbers and methods. His serial killers aren't all sociopathic spy-flick supercriminals who kill for pleasure (as they tend to be in movies). They are often disorganized killers, such as schizophrenics, who sometimes believe they have valid reasons to snuff out lives. (Ressler is also the person who created the organized/disorganized typology.)
The serial killer is sequential and finds victims over time; the mass killer is multiple and rapid. These are the main distinctions in the original sense.