Tiberius was an Emperor of Rome for a short period between Augustus and Caligula. He was murdered by his nephew Caligula alongside many of Caligula's family members, a fact that alarmed the archangel Michael and prompted him to ask Xena to stop his reign of terror.
Given Xena's statements about "run-of-the-mill" Roman Emperors, it's likely that Tiberius was also "a psycho, a sex addict, and a murderer."
In reality, Tiberius was the son of Livia, and would thus be Xena's grandson if the writers stuck to the historical record. For rather obvious reasons, this relationship was left out. Historically, Tiberius was indeed a psycho, sex addict and murderer, although apparently to less of an extreme than Caligula.