Killing people is what they're for. Killing people is the point of most guns. Most people target shoot. Some people kill in self defense. Guns that are really good at killing are also really good at self defense.
When Seung-Hui Cho can kill 30 people with just handguns, I don't think it's "assault weapons" that are a problem enabling people to kill more easily than other kinds of guns. Anyone defenseless is going to be at the mercy of someone with an effective weapon. Let's not even get into the politics of the word invented solely to classify broadly a set of guns to ban in the late 1980s.
People want "assault weapons" for when two people break into their home at night, maybe armed, but you forgot to ask them when they came in through your window at 2AM. Do you want a 6 shot revolver, shooting in the dark at both of them and a reload while you shake and just woke up out of bed where you drop loose cartridges all over the ground, reaching in the dark for them? Or do you want to be able to shoot 30 times, knowing you will miss most of the time, when police hit less than 1 in five shots fired, and the capability to reload as easily as sticking a VHS tape into a VHS player and pressing "play?"