I would say the terrorist is likely to have the highest body count in the end.

Do some rough math on 9/11. 9 terrorists and 2977 deaths (331/terrorist).

The most prolific mass murders have "alleged" death tolls in the hundreds, but most are only multiples of ten.

That is average number spread out over all the terrorists and not median. Trying to merge those two concept is a common way to use statistic to mislead.

The question was if a shotgun wielding guy posting a video on Youtube is more likely to kill more people than a shotgun wielding mass murderer who robs liqueur stores. To clarify, he doesn't have a time machine to suddenly transfer himself to 2001.

Not fair; the public was not prepared to deal with that situation; now they are. Pilots carry; attendants and passengers are knowledgeable to fight back. And that's including revoking most of what the TSA has become.

Also, the cockpit doors are reinforced.

That's mostly a consequence of planes and skyscrapers having lots of people.

The death count could as well be seen with a place accidentally crashing into the Empire State Building.