The plain reality is that he would have not have been able to inflict such carnage if he did not have that weapon. I am amazed that US citizens fail to grasp such simple facts.
BTW, I am also curious: why do people need an assault rifle? I can understand a collector or dealer would want to be able to own one but why an ordinary person?
I think Timothy McVeigh and some guys from Saudi Arabia would beg to differ on how much carnage can be done without firearms. Don't underestimate motivated sociopaths.
An ordinary person doesn't need an assault rifle but a car buff doesn't need cars either. They just genuinely appreciate their design and performance, especially ex-military personnel. Guns in the US are tied in with our identity more than most countries; the idea of the armed citizen is in our cultural DNA. And overwhelmingly, their owners care for them responsibly and don't hurt a soul.
Without wishing to put words into Obviousman's mouth, while it's true a motivated sociopath can do a lot of damage without guns, it's just a darn sight easier to cause mayhem with a gun. Specifically, you need less than an hour and, what, a couple of thousand dollars to go to your local gun shop, purchase a few firearms and some ammunition, then go out and kill dozens of people. By contrast, the 9/11 attacks took years of planning and must have cost hundreds of thousands of dollars in air fares, flying lessons, hotel bills and so on.
The danger with firearms is the damage which can be caused by someone who's just Having A Bad Day.
Back in the late 1980s Australia had a couple of massed killings caused by people with guns, with six to eight people killed. These were obviously leading news stories of the day (given the population difference between Australia and the USA I'll let you calculate what the equivalent casualties would be). But I also very vividly remember reading a story at the time, on about page 5 of the newspaper, only a few centimetres long, of a man who went on a rampage in a shopping centre with a knife: something like seven people were wounded. And I remembered thinking,
at least he didn't have a gun, otherwise it would have been another mass killing. Very simply, a gun allows you to kill people tens of metres away with just the twitch of your index finger, even if they're running away; with a knife you need to be right next to someone, and you need to use your whole arm: physically and psychologically it's a lot easier to kill people with a gun than with a knife.