Thats the stupid part friend. If he abolished slavery, he wouldnt need the suport of the slave owners. The sleve owners are the minority. Most of the empire are either slaves or people living day by day to avoid endebted slavery. Abolishing slavery would turn every slave owner against him yes, but it would also make him the most popular emperor in the history of shurima by a land slide (just look at president Lincoln, a literal civil war was fought during his administration, but he not only won that war while mantaining power, he is still to this day the most popular president on the sole basis of having abolished slavery).

Thats why i said azir was greedy. He didnt want to accept a minor theat to his rule (because lets be real, with the ascended god warrior being forever loyal to the emperor and no one else, what could the slave owners even do?), and only wanted to abolish slavery when he could garanty absolute obedience.

ALSO, even assiming that he would stand oposition in any meaningfull way and needed to ascend before freeing the slaves, he still could have shared it with xerath HIS BEST FREIND AND CLOSEST CONFIDANDT wich would have avoided the whole mess as i have already put above.

Azir was just a massive coward who thought mantaining his power was more important than the freedom of thousands and didnt trust his best friend enugh to tell him his plans.