“That is not necessarily true. A terrorist commits the deed as an expression of a political or religious goal or mission; a mass shooter may be a terrorist, or he may just be a lunatic who commits his action without any coherent rhyme or reason, or just for the sake of sadism.”
If we accept mental condition as a contributing factor for a person that commits a mass shooting, we will be faced with a situation were a Islamic bomber claims the “Insanity Defense.”
“John Wayne Gacy was a mass murderer....”
John Wayne Gacy and Ted Bundy were serial killers, not mass murderers.
“Adam Lanza was a lunatic who was on prescription drugs...”
Adam Lanza being on prescription drugs for an emotion disorder is irrelevant to his actions.
And the reason it is irrelevant is that I fully expect we will learn that the Boston Bombers were also on prescription drugs.
I think you will find little distinction between a serial killer and a mass murderer. The time element is arbitrary.