Why does aizen want to kill the soul king
He knew the four four ever since the fall of the Shiba clan royal families were the ones actually ruling Soul Society and that the Chamber of 46 were nothing more than tools of the royal families. So Aizen's goal was basically to take the Soul King power for himself. And destroy that sham, that facade, that was fooling everyone. For example. In the novel we also learn that Yhwatch wasn't really defeated, not permanently.
When Ichigo cut him in half, Yuha was already regaining the power he had stolen, and thus he became the new Soul King. Trapping Yuha in the crystal the previous Soul King had been. If I'm not mistaken they even cut off some of his limbs and spread them in different parts of the different worlds in order to make the seal more efficient.
But we are also told by Uryuu that the seal will only hold Yuha Soul King for one hundred years at best, but he would eventually break out. We are not explained how Uryuu knows this only that he knows it. If half of what was in the novel was something Tite Kubo was actually gonna explain if he had had the time, then we can say that Aizen was not really the villain we all thought he was.
That Soul Society were far worse! Seeing that if Ichigo hadn't been delayed by his training then he would've been tricked by the Zero Division into taking the power of the Soul King to defeat Yuha and then the Zero Division would've sealed him and did the same to Ichigo. His two swords show his actual strength. The shorter sword being the quincy one, the bigger one being the soul reaper power. Why does Aizen hate the soul king? Why does Yhwach hate the soul king?
Is Yhwach stronger than Soul King? Is Ichigo stronger than the soul king? Does Ichigo ever kiss Orihime? Did Ichigo get weaker after Soul Society? Why is Ichigos bankai so weak? Is Ichigo stronger in the Soul Society? Why is Ichigo so weak in the Arrancar arc? Interestingly, the Soul King can understand what's going on around him, but he is unable to respond or show any form of expression whatsoever.
There are five Shinigami given the special honor of protecting the Soul King and his so-called family that has never been introduced. This quintet almost never leaves the Royal Realm, relegating many tasks to Seireitei's Shinigami. The Royal Guard also has direct access to the Soul King by means of their bones, within which a technique known as Oken has been integrated. Both the Soul King and Yhwach manifest eyes containing four irises located equidistant from one another.
The Quincy King uses this complex set of optics to bring about the Almighty, with which he can perceive as well as select any future timeline he wishes. It's possible that the Soul King possesses these powers as well, but they are not described in the narrative. However, it explains why Yhwach refers to him as his father.
Presumably intending to take his father's place, Yhwach travels to the Royal Palace to finish off the Soul King. After killing the latter's attendants, the Quincy King mocks his father, calling him too feeble and incomplete to defend himself. Yhwach stabs the Soul King through the chest, before being interrupted by Ichigo and team.
Nevertheless, he is confident that the heroes will fail before him, because his Almighty has proven that the death of the Soul King is inevitable. Ichigo attempts to pull Yhwach's sword out, but this is the point when the young hero's Blut Vene is triggered — and Ichigo unwillingly bisects the Soul King.
Yhwach then reveals that anyone who has Quincy genes will not be able to abide by the Soul King, and that they would be naturally drawn towards killing him. To make things worse, Orihime's god-level technique, Soten Kisshun, collapses when she tries to heal the Soul King.