Does being nicknamed the devil count as worst, if so there is:
Robert I, Duke of Normandy (French: Robert le Diable)
Vlad II of Wallachia (Romanian: Vlad Dracul)
But it is unlikely to have a monarch nicknamed the worst as for example, who is the worst monarch of England and who decides? To the catholic Mary I and James II were great whereas Elizabeth is a heretic.
It alright picking one, but whose not to say their successor wont be worse or a monarch ten generations from now? eg. Henry VIII could be seen as the worse, but then what about Mary I and Charles I.
Then what do you nickname them, King Henry VIII, "the not-so-worse-then-Charles-I-but-still-worse-then-others"? Not very catchy, where as calling "the Great" can be used as many times.