If you have never read Tacitus, then I suggest that you do. He is a master historian with a critical eye when it comes to analyzing people. Many people have seen in Tacitus a strong republican inclination. By this, these people mean that Tacitus was against the principate and tyranny. To put it in other words, Tacitus was for the republican form of government in Rome.
Right from the beginning of the Annals, Tacitus recounts the fall of liberty and freedom. After briefly going over Augustus, his work focuses on Tiberius, who, according to Tacitus, is a pathological tyrant, who introduces a reign of terror. He is also perverse. The loss of liberty is certainly one of his primary points. He also sees in the other emperors the same impulse towards tyranny, especially in Nero, who is characterized a completely debauched and insane.
From this perspective, Tacitus is not in favor of the developing trends in Rome, namely, the empire. In my opinion, as far as we can tell, Tacitus is a republican historian.