Wrong. Can you honestly not even consider any other purpose that someone would want one than killing people? These things get bought outside of America too, the guns you see at gun clubs in the US are the same as the ones you'll see here, and nobody buys them to attack people in either country.
I can buy all the same assault weapons that an American can. What I can't buy in the same way are handguns, and those are what make up the bulk of the gun deaths in the US. What I find strange is that you're reasonably OK with handguns for personal defense, yet they're used for murder more than any other method in the US. It seems odd to me when the discussion hinges on purpose built for killing, it becomes centered around guns that are a drop in the US gun violence bucket (we're talking ~300 murders using rifles and 6000+ with handguns).
So where is the line drawn when it's too powerful? The way you're using high powered suggests to me that you don't know how this stuff all works. The round an AR-15 uses is actually fairly low powered in the grand scheme of things, substantially less powerful than the rounds that any of the standard issue rifles in WWI used.Most people wouldn't even consider it enough for deer hunting, they'll use shotguns or a .308 (M21) or a 30-06 (M1 Garand). Get into moose, elk, or bear hunting and people regularly use even more powerful rounds.
Civilians use AR-15's for hunting, defense, and sport shooting for various reasons. Mostly it's a combination of the accuracy, low recoil, and light weight of the rifle. Two of those 3 are the qualities of the round it fires, and that doesn't change whether it's an AR-15 or one of the hundreds of other guns that use the same round (both sporting and assault weapon type). AR-15's in particular are a convenient platform for a sport shooter because accessories and parts are affordable and easy to work with. The overlap in this case is that military and sport shooting (and defense) uses are similar in their requirements.
Once again, it's the Honda Civic of the gun community, right down to there being an equivalent to the V8 muscle guys that think they're not powerful enough and a real man uses a .30 calibre. All the pistol grips, foregrips, flashlights, lasers, and other "tactical" stuff that you'll see attached to them are the fartcan and ricer bodykit of the gun world. It's often just stuff to look cool with mild practicality, just like ricers want their unmodified cars to look/sound fast, and they don't make the rifle any more powerful or capable.
I'm not trying to just go gun nerd on you here and argue about semantics. I'm bringing up specifics because they're important to this discussion. There are a ton of misconceptions about assault weapons and their capabilities that can influence this kind of discussion. Think of a round as a car's engine, just as the same engine might go into a small car, full size car, and minivan, the same round might be used in an assault, bolt action, and other types of rifles.
