It's the little piece of lead that egresses the hollow tube at an alarming rate of speed, such that it pierces the skin and causes massive shock and loss of blood that kills people. No, wait, that's not it... it's the person who pulls the trigger that kills people. No... still not right. It's the gun shop who sells the gun to the person who pulls the trigger... wait, we're missing something here... that must be it! The company that manufactures the gun kills people.
At least, that must be the crux of the argument of those who cheer the Supreme Court's refusal to block lawsuits against gun manufacturers.
The supporters of the suit point to the accountability that gun manufacturers must have in knowing that they make a tool that is deadly if misused even slightly, and deadly if used properly for the wrong reason. Detractors of the suit feel that the suit interferes with manufacturer's right to sell their weapons, claiming damage under the Commerce Clause (NOT the second amendment).
The suit that is allowed in deals specifically with assault weapons. And it deals with gunmaker liability under the tort of negligence for violence in and around the Nation's Capitol. The argument is that D.C.'s law (that would allow the suits mentioned above) interferes with gun commerce in other states.
Let's consider something for a moment. Let's assume Maine passes a law that holds automobile manufacturers liable for drunk driving fatalities due to manufacturer's negligence. The negligence argument is that car manufacturers are making automobiles that can be deadly if misused, and as they know that drunk driving can kill. Now, the lawsuit that would be filed would argue that the law impedes the sale of cars in states outside of Maine due to Maine's restriction on commerce, which is illegal under the Constitution. Nobody is arguing about the right to own cars, they instead argue that the car makers should be liable for damage caused by the cars that people bought with full knowledge of their deadly potential. The Court of Appeals hears the case and says that the argument is unpersuasive because there's nothing that shows the statute interferes with someone buying a car in Arizona, or Montana, or Ohio, or whatever.
Of course, nobody would pass a negligence statute on car manufacturers, so we don't have to worry about it. I don't know that this statute would pass muster, though, if it didn't deal with assault weapons and handgun manufacturer liability.