The first and most famous accusation towards Nero is that of being a horrible person, even before being a bad emperor. The ancient historian Suetonius has no kind words in describing his personality.
Nero is depicted not only as evil but also as crazy, lustful, greedy, and cruel. Not, indeed, a person you would like to be friends with.
Well, at least apparently. Where’s the trick here, you say? Suetonius and the other most famous historians — Cassius Dio and Tacitus — were all aristocrats. Nero’s politics was centered around the needs of the lowest classes, not the highest ones. Combine this fact with Nero’s hate towards the senate, and you have your motivation for his biographers’ hostility.