And people don’t just stop because you shot them once, or twice.

Yes and no. A lot depends on both shot placement and the firearm being used. Centerfire rifles with bullets traveling more than 2200fps (roughly; some estimates say 2800fps) will stop a person much faster than a pistol, since the temporary wound cavity becomes a permanent wound cavity. But that's going to be true for nearly any centerfire rifle, aside from old cartridges that were designed around black powder (e.g., .45-70); an AR-style rifle isn't going to be more lethal than any other fast-moving centerfire rifle cartridge, it's just a fairly lightweight and easy to use rifle compared to grandad's M-1. Pistol cartridges can stop threats as effectively as rifles, but you require better shot placement, and you generally want to have defensive (e.g. hollowpoint) ammo. (There's a reason that "failure to stop" AKA Mozambique drills are a good training tool.)

So with that in mind, shooting isn’t easy

A rifle is, for an able-bodied person, easier to shoot accurately and effectively than a pistol. Part of that is because you have a longer sight radius, and part of it is because you have a shoulder brace (...and a pistol with a shoulder brace is a short barrelled rifle, which is generally illegal without the BATF gittin' all up in yo shit). It's pretty easy, relatively speaking, to hit a target at 100y with a rifle, and very difficult with a pistol.

many countries in Europe have civilian gun ownership laws

Eh. Civilian gun ownership is difficult and expensive in many European countries. However, many European countries do have combined violent crime rates (defined as murder, robbery, forcible rape, and battery) significantly lower than the US. Violent crime, in general, is lower in Europe. So it's not just that gun crimes are less likely, but that you're also less likely to be sexually assaulted, or get jumped and beaten. There's almost certainly something different going on in social conditions that make violent crime less likely, and that would make it less likely even if European countries had gun laws that were more relaxed.