Unlike members of today's society where the law protects the public freedom to criticize the government, Tacitus lived in a dangerous city where saying the wrong thing could mean execution. That's what makes his Annals so genuinely fascinating, because he seems to be doing the typical "honor the emperor" song and dance (especially for the first two emperors, Augustus and Tiberius), but when he gets to the current emperor, Nero, he openly criticizes Nero's corruption and violent authoritarian habits.
Basically, the book seems to be an explanation of the downfall of the state as it devolved from a Republic with a senate, to a dictatorship where the senate is filled with yesmen who tell the king whatever he wants to hear. The people's interests are completely disenfranchised by this, and the senators are all fat, happy, and rich. Their constant support of the emperor is rewarded greatly, especially in Nero's reign, which Tacitus despises.
Nero's reign is the central issue of the text because Tacitus is obviously highly disturbed by Nero's methods and standards. Nero is someone who, according to Tacitus, doesn't have any regard for public opinion. His methods for leadership are Machiavellian at best, and at worst, they are downright cruel.
Tacitus is basically serving as a journalist, committed to telling the truth, regardless of how it might harm him in the future.
