There are a LOT of semi-automatic rifles that aren't AR15s. There are semi-automatic shotguns, and semi-automatic pistols as well. Speaking objectively, most firearms bought today are semi-automatic. A semi-automatic weapon is inherently no more or less dangerous than any other gun in the hands of someone who knows how to use it.
Just for clarity, and I don't mean to assume that you don't know, but as most people I've spoken with have it entirely wrong, a semi-automatic firearm means that when you pull the trigger, a bullet is fired. There is no 'burst' or 'spray' mode, and there's no tangible difference between an AR15 and, say, a Browning BAR (http://www.chuckhawks.com/browning_BAR.jpg), and "assault weapons bans", in my opinion, penalize a gun for looking like military weapons, when in reality they function nothing like military weapons (as they don't have burst, spray or fully automatic fire modes).
If they're famous for causing havoc recently, it almost certainly speaks more to their popularity than anything else. If the most popular car in the America were the Ford Taurus, it would likely be involved in more accidents than other cars, based on sheer math. The AR15 is the most popular rifle sold in America today. The reasons for its popularity are because of its modular design, not its lethality. With an AR15, I can replace the upper assembly without need of a gunsmith. This means I can have one "gun" for both long and short range shooting. I can 'bolt on' attachments like a scope, night lights, laser sights, bipods, etc., without having to pay somebody else to do it for me. It's popular today for the same reasons Linux is popular with nerds -- because you can tinker with it.
As for its lethality, it fires a small caliber, low-weight round (.223). In its generally short-barreled configuration (though again, that is moduler) it is considered a 'varmint' gun, used to kill raccoons, coyotes, wolves and the like from a ranch, or what have you. If people were choosing their killing weapons based on lethality, they would have chosen an AR10 (.308 round) or something like a Saiga 12, which is a magazine-fed shotgun.