KEY THING TO REMEMBER.
Azir's abolishment of slavery happened mere MINUTES before his death and Shurima's destruction. This means that the only people who witnessed the event were Azir, who died and was only ressurected in recent times, Xerath, who spent a millennia in a tomb with Renekton, and the population of the capital, who all died instantly as Xerath's plan unfolded.
The story is incomplete for just about everyone in Runeterra, and so the details known about the return of the Empire by the wider population of Valoran are going to be extremely limited and based upon what was left behind: the myths, legends and history of Shurima, and given how Azir's father was described by Xerath as being essentially a mad man, it's safe to say the Emperor was not viewed as a shining example of excellence all the time, and the stories that endured throught he ages were of the tyranny of the Emperors, not their benevolence if they even had any. Azir, being the last emperor and someone partially blamed for its fall because of his own hubris, will also fall under scrutiny because of the stories about him and the legacy of the emperors.
Only Azir, Xerath, Nasus and Sivir really have any real idea of what truly happened back then. Four people in the entire world. I wouldn't be surprised if Azir's return as an Ascended sparked fear and terror across Shurima as people feared the wrath of the returned emperor, and in a part of the world like Ionia, which is a continent in its own right, the details would be even muddier.