I vote for Andronikos I (Komnenos), Manuel's wayward cousin. Seized power 1182, killed young Alexios II in 1183 having already killed Alexios's sister Maria, her husband Renier, Alexios's mother Xene, and lots of others; killed many more before he was himself killed, horribly, by the people of Constantinople, in 1185. He only had a short time to rival Nero, but he worked hard at it. Oh, and (aged 65) he married Alexios's fiancee, Agnes/Anna, who was then aged 11.
Andrew