Astana was probably too young to be emperor. He’s also dumb and lazy and it’s easier to oust a young, stupid, irresponsible king than an adult one. And she probably has more control as empress than as mother of an emperor. Especially because the current emperor is useless and kinda just lets things happen so the empress gets away with a lot of the evil shit she’s been doing.
iirc there was also a chapter where a grown-up Astana began to answer back to the empress so maybe she has trust issues with him now and if he becomes king, she thinks she won’t be as powerful as before.
Orrr it could be as simple as: Before, the emperor was more useful alive. Now, he’s more useful dead.
If the authority to appoint the emperor would fall to the empress following the death of the emperor, im curious as to why she didnt kill him sooner? Why did she wait so long i wonder?