I'd have to do something that people would talk about with hushed voices for decades to come... and those directly involved would be seeing therapists for years. I can't quite work out a way of killing a large number of people that is both horrifically violent and quick. Of course, you couldn't just use an indirect weapon - it'd have to be very personal, to enhance the horror of it all. Butchering somebody with a knife in front of their children is going to leave a far more lasting impression than blowing the whole family up with a grenade. I guess running round a shopping center with a knife, slashing abdomens open and pulling out the stuff inside (while the victims are still living, of course) would have the desired effect - especially if you made sure you only did it to every other person, so there were plenty of people to live with the trauma. After all, you're not going to cure cancer in 60 minutes, and the way things are going lately I'm sure I'd be forgotten sooner or later...