He didn’t fiddle while Rome burned (since fiddles hadn’t been invented), but the decadent emperor did watch the fire, which he later blamed on a new religious sect called “Christians.”
He began Rome’s first official persecution of the faithful, and tradition says that both Peter and Paul were martyred under Nero.
Ironically, the reason Paul was in Rome was that after his arrest in Jerusalem, he demanded to have his trial in Rome.
He used the legal formula available to a citizen of Rome: “I appeal to Caesar” (Acts 25:11). The caesar (emperor) he was appealing to was Nero.
Nero’s immorality and cruelty were notorious, and people’s revulsion at Rome’s moral decline made them receptive to the moral standards of Christianity.