You are eliding the (far more common, if usually settled in less high-profile settings) case of people who show up with a gun, not with the intention of just spraying rounds into the crowd until the cops take them down; but because they are looking to kill somebody in particular, for reasons of some personal grudge or something.
If your sole criterion is 'shoot as many people as possible because $SOMETHING$', sure, you start at a distance suited to your weapon near the densest unarmed crowd and begin shooting. If you actually want to kill somebody in particular, though, that's about the stupidest thing you can do. You get as close as you can, without attracting attention, and ideally make your kill and leave.
The odd thing about this case is that it falls awkwardly between the two: If you have a personal grudge, an international airport is crowded public, and going to have the cops on call on very short notice. Just look up the guy's home address.
If this was a generic spree killing, it was a remarkably tepid one.