CHARLESTON, S.C. — The Stephen Colbert “super PAC” is now officially legitimate: it’s running an attack ad. In a 60-second commercial that is being broadcast on the CBS station here, Mr. Colbert’s super PAC calls Mitt Romney a serial killer. Yes, you read that correctly. “Mitt Romney has a secret,” says the announcer, whose deep, distinctive voice people will recognize as John Lithgow’s. “As head of Bain Capital, he bought companies, carved them up and got rid of what he couldn’t use.” Recalling Mr. Romney’s remark that “corporations are people,” the commercial concludes, “He’s Mitt the Ripper.”