He designed a selective-fire rifle with some useful features to improve reliability, portability, etc. When he designed it, it made no sense to think of civilians using it. Civilians still have a heck of a time legally getting a selective-fire AR-15 or its offspring, the M16.
It was redesigned a bit a few years later to make it semi-automatic only. As such, it's generally as lethal as any other semi-automatic. Semi-automatics have been around and been fairly common for a long time.
I don't know what "Some gun control advocates have argued that assault rifles were never intended for civilian use and should be banned" means.
On the surface, it's a trivial statement: Compare "Tanks were never intended for civilian use and should be banned." Perfectly true. But it's pretty much the (de facto) case with assault rifles for the last 80 years, if by "assault rifle" we mean "selective-fire rifles."
Under another reading, it's pretty meaningless: "Scary looking, military-style weapons were never intended for civilian use and should be banned." Fine, ban aesthetics. Here "assault rifle" just means "scary, military-style" weapons--not "military-function" weapons. It's saying form is more important than function, a common failing these days. Perhaps if the AR-15 were robin's egg blue or some other nice pastel it wouldn't be as objectionable, even if its function was the same. Now, there are scary looking, military-style toy guns that should be banned, but in that case because they're so realistic they can get a kid killed if he points it at a cop, but if a kid's killed doing that it's assumed to be the cop's fault. Because in the split second the cop decides, all he can see is form and from that has to infer function. Both this reading of the quoted sentence and the cop's misjudgment put form over function, but one has the luxury of time to get past that.
In yet another reading, it rewrites over 100 years of history and misses some big points: "Anything more than single-shot weapons were never intended for civilian use and should be banned." I don't know that the inventors of semi-automatics had any intentions as to civilian use and doubt that the producers of that statement do, either. Semi-automatics have been around for quite a while. Some of the earliest ones were produced for civilian use, but the invention had been around for a decade or two before that: In the absence of an on-going or imminent war and given other technical issues, some manufacturer designed and produced semi-automatic shotguns.
Note that even rifles with bolt action were originally designed for the military, but that's what gun-control advocates fall back to--a military design repurposed for civilian use. Rifling was also for the military, too. Military is the single biggest customer for firearms. You make a new product on spec, you can try to sell 5000 to a single buyer or you can come up with an advertising campaign, distribution network, put in the money to produce stock that you store until you can ship it.
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