There are plenty of other contenders for worst Roman Emperor – Nero and Commodus for example – but Caligula's mad reign sets a high standard. After a promising start to his reign he seems to have set out specifically to intimidate and humiliate the senate and high command of the army, and he gave grave offense, not least in Jerusalem, by declaring himself a god; even the Romans normally only recognized deification after death.
Caligula instituted a reign of terror through arbitrary arrest for treason, much as his predecessor Tiberius had done; it was also widely rumored that he was engaged in incest with his sisters and that he lived a life of sexual debauchery, and this may well be true. The story of his making his horse a consul, meanwhile, may have been exaggerated, but it was not out of character.
Caligula's unforgivable mistake was to jeopardize Rome's military reputation by declaring a sort of surreal war on the sea, ordering his soldiers to wade in and slash at the waves with their swords and collecting chests full of seashells as the spoils of his 'victory' over the god Neptune, king of the sea and by his failed campaign against the Germans, for which he still awarded himself a triumph. He was assassinated by the Praetorian Guard in AD 41.
Caligula's successor, Claudius, was an improvement but, despite the favorable picture in Robert Graves's famous book I, Claudius, not by much.
Pope John XII (954–964)
Even by the lax standards of the medieval papacy, John XII stands out as a disaster of the highest order. He was elected pope at the ripe old age of 18 as part of a political deal with the Roman nobility, and he inherited a conflict for control of Italy between the papacy and the Italian king Berengarius.
John had the support of the powerful German emperor Otto I, who swore to defend John's title from him, but John himself was too taken up with a life of drunken sex parties in the Lateran to care too much either way. He recovered from his hangover enough to accept Otto's oath of undying loyalty and then promptly linked up behind Otto's back with his enemy, Berengarius.
Understandably annoyed, Otto had John overthrown and accused, among other things, of simony (clerical corruption), murder, perjury and incest, and he replaced him with a new pope, Leo VIII. However, John made a comeback and had Leo's supporters punished ruthlessly: one cardinal had his hand cut off and he had a bishop whipped.
Full-scale war broke out between John and Otto, until John unexpectedly died – in bed with another man's wife, or so rumor had it.