Honest question: why would you even post a comment like that when searching online for the answer takes like 10 seconds?

The fact that you want to go with your feels and that you have the balls to degrade someone else who was actually correct is really what says everything we need to know:

https://en.m.wikipedia.org/wiki/List_of_mass_shootings_in_th...

FWIW your "reasonable definition" of mass shooter requiring the victims to be unknown by the shooter seems totally unreasonable to me, and it's not used by any organization that actually tracks these things (the Wikipedia article gives a list of definitions, none of them conforming to yours).