My wife has been binge watching it so it is on the top of my head right now.
But if you believe some of the stuff on the show (which you have to take with a large grain of salt) the mindset of a serial killer vs a mass killer would be very different. But like I said, I'm basing that analysis on a fictional tv show so I could be 100% wrong.
I would also that you are right about being a serial killer today. It is much more difficult.