He was a 2nd-century B.C. Han prince. He is one of the earliest serial killers attested by historical sources. My questions are: how many people did he kill, why did he kill them, and is he depicted in the Book of Han?
According to the book of Han, it's 100 plus some (百餘人). The full entry is as follows and pretty much matches the description on Wikipedia:
The translation on Wikipedia is acceptable so I'll reproduce here (with slight modifications to match the original text, emphasis mine):
twenty-nine years later, he was arrogant and cruel and would go out on marauding expeditions with tens of slaves or young men who were in hiding from the law, murdering people and seizing their belongings for sheer sport. Confirmed victims exceeded 100, and these murders were known across the kingdom, so people were afraid of going out of their houses at night. Eventually, the son of one of his victims accused him to the Emperor, and the officials of the court requested that Liu Pengli be executed; however, the Emperor could not bear to have his own nephew killed, and Liu Pengli was made a commoner and banished to the county of Shangyong, his sovereignty was abolished and his land was reclaimed as the Dahe commandery.
Do note that he did not kill this number alone; he had slaves and goons accompany him.