The FBI & one of their former top profilers; Robert Ressler define a "serial killer" as a subject(s) who kill using the same way with the same weapons-methods at different times. Serial killers must also have killed at least three victims.
Ressler, a former US Army officer(0-6/CIDC) , wrote a few non fiction books on criminal behavior & serial killers. To me, he's more creditable than John Douglas.
Douglas, a retired FBI special agent & profiler wrote about a lot of things the FBI either quit using or was later shown to be unrelated/useless.