WHO KILLED PISO AND WHY? For Mary Beard, a distinguished Cambridge classicist, that nagging question persists after perusing Tacitus. Of course, that was only one of the many deaths in the "Year of the Four Emperors," 69 AD. This review/essay from the current issue of the London Review of Books reminds one of the delights and horrors to be found in both the "Annals" and the "Histories."