No, the full-auto selector is not the villain here, its just a red herring. I certainly agree full-auto fire capability should be illegal, but it is not what really defines an assault rifle employed in mass murder mode.
I think magazine capacity is the primary characteristic. No one would buy an AR-16 or an AK-47 for “hunting” if it only came with a 5 round magazine, like most hunting rifles.

Gun owners and fanciers don’t have any trouble in coming up with specifications for a long gun suitable for target competition, hunting (both large and small game), combat (and there are several types in that category) and so on.

But if I wanted a gun specifically for killing large numbers of civilians in a crowd and creating terror, I would pick something like an AK-47 or AR-16. It is easy to carry, has moderate recoil, it is effective at close to medium ranges (its not a sniper weapon), and you can carry lots of ammo.

A fully automatic fire mode would not be necessary, that is not useful for spraying a nearby area to take out a lot of people in a hurry (unless they are packed close together, as in a rock concert). For enfilade fire sweeping out a trench full of enemy soldiers, or firing blindly into a confined space where you suspect someone is hiding, you need the full auto switch. This is why the submachine gun, or machine pistol, was invented. It was usually chambered for pistol ammunition, like the US Thompson .45, or the British 9mm STEN gun or the German “Schmeisser”. This application doesn’t occur very often in real warfare (soldiers dismiss it derisively as “killing a man in a phone booth”).

You can use an assault rifle in this way, but those weapons are more likely to be used to fire at more distant area targets, like a building a few hundred feet away, or a tree line. The idea is to simply force your enemies to keep their heads down and not shoot at you. It prevents them from maneuvering and pins them down until you can call in artillery or an air strike. You’re not very likely to actually hit anybody. In fact, suppressing fire like this is not very effective at inflicting casualties unless an entire unit, like a squad or platoon, is doing it. The ability to mow down a line of charging infantry with full auto fire is only useful in the movies. You’re better off picking them off one by one.

Full auto fire may come in handy if you’re using the weapon as you would a shotgun, like you walk around a corner and find yourself facing an armed opponent–you don’t have time to do anything but empty a full magazine in his general direction, but unless you’re part of a small infantry unit packing lots of ammo, it simply isn’t a very realistic tactic at the ranges you are likely to engage an enemy.

Full auto fire is usually reserved for weapons like the Browning Automatic Rifle, or the British BREN, a heavy (20 pound), 20 round portable machine gun used to return accurate, high volume, full rifle cartridge fire at a fixed position, possible very far away. In WWII, each fire team was equipped with one, the BAR man would pin down a machine gun nest, providing covering fire while the rest of the squad moved in and outflanked it, finishing it off with grenades.

In Vietnam, the otherwise excellent American M14 was a full size infantry rifle with a full auto switch, but in the jungle or urban warfare, it was cumbersome and awkward, so the M16 was preferred for close-in work.

To sum up, the M16, or AK-47, is perfectly optimized for shooting up schools, movie theaters, public places and so on. And it can still be used at moderate distances, It can fire 20-30 rounds without reloading, it is easily reloaded by inserting a fresh magazine, it is accurate and lethal at a hundred yards, even in the hands of a novice, and the ammo is light–you can carry lots of it. It is also light, compact, and can be easily broken down and concealed. It doesn’t need an auto selector switch, but a bumpstock or a quick modification by a gunsmith can take care of that problem. That whole selector switch dodge is a scam to make us think this problem is easily solved by a quick fix or legislation, so we can keep on selling these guns to killers and wannabees.

To stop mucker shootings like this, the best thing we could do is simply outlaw all weapons capable of utilizing high-capacity, replaceable clips. You don’t need 30 rounds, and five spare magazines, to hunt deer. The full auto selector switch is not the killer, its just a distraction to make us believe it is.