History Memes

The Praetorian Guard weren't completely disbanded, as an institution, until the reign of Constantine I around 310 AD. At that point, though, their function as secret police was less relevant as the Emperor increasingly ignored the Senate and rarely stayed in one place, instead moving with the imperial court, and a series of other bodyguards were implemented in their place. By then the Empire itself was falling apart, though.

However, numerous Emperors with strong military backing purged the Praetorian Guard after coming to power - in whole or in part (those Emperors who came to power without strong military backing generally had no opportunity to act hostilely towards the Praetorians if they valued their head). The ones who did so were generally free of the Praetorians making such demands of them - either because they had been replaced with the new Emperor's partisans and veterans of loyal legions who were thrilled to get a cushy position far from the frontier, with extra pay, lax discipline, and an early retirement; or because the remaining unpurged Praetorians knew that the regular Legions, who were generally very loyal towards Emperors who had a long history of military service, would take a brutal vengeance against the (widely hated) Praetorians if they tried anything.

A big part of the reason why the Praetorians remained despite everything - including the fact that there were other military units who were supposed to protect the Emperor - was because the Praetorian function as a secret police was extremely useful in the cutthroat politics of the Empire. The Emperor was not a particularly special soul - the Empire was an (effective, if not nominal) autocracy, but not really a monarchy for first ~300 years of its existence. There was no special legitimacy to the occupant of the office, and the early attempts of Augustus to establish a divine mien were weakened by the fact that... every fucking major family in Ancient Rome claimed descent from half-a-dozen gods to begin with.

The Emperor was an office, and offices are always ripe for new occupants to take their place. Senators very often had their eye on claiming the position for themselves, and an Emperor who did not root out such conspirators could end up massacred by non Praetorian conspirators, like how the Emperor Galba fell, and how the Emperor Domitian almost died (Domitian was a dick though, he would've deserved it). The Praetorian Guard could, at least, pretty reliably be bribed due to the fact that they were largely not of the appropriate social status to become heavyweight politicians, and so could not benefit from putting their eyes on the higher goal of the imperial title. Senators, on the other hand, could be... more troublesome in their ambitions.

The Praetorian Guard's nominal goal was the protection of the Emperor - but they generally did so not by open combat or even suppressing riots (that was the function of another unit, the Urban Cohorts, which was, by contrast, not noted to be particularly disloyal), but by eavesdropping on Senators and other notables, secretly reading their documents, intimidating those who seemed like they might be planning something and, in extreme cases, extrajudicial abductions and executions. Secret police bullshit. Ugly stuff.

For example, the Emperor Vespasian appointed his son Titus, who was known as a merciful and generous man, to the position of Praetorian Prefect. Either because the position of chief of secret police is pretty inherently fucking damning, or because he was protective of his old man (whom he was noted to be genuinely close with), Titus reversed his previous reputation as a (by Roman standards) gentle (if somewhat party-boy) soul to a feared figure, once even having a suspected Senator conspirator's throat cut in the hallway just after the Senator left a banquet with Titus.

Titus would go on to be an extremely merciful Emperor who was widely beloved (and somewhat surprised the terrified Roman people by the change in his demeanor from his time as Praetorian Prefect), which either speaks to an immense interest in public relations and an equally immense amount of self-control and dissimulation to go from behaving as a brutal thug to behaving as a genuine, lenient, and merciful benefactor (regardless of which was 'more' him by nature); or the immensely fucked nature of even 'good men' in the role of a secret police chief.

Perhaps both.