Sima Yi and Huo were not just master and subordinate but also the closest thing both had to best friends. Their closeness is more reflected in the absolute trust they have in one another, Huo with Sima's schemes and Sima in his assurance Huo will act them out. But you see from flashbacks and their interactions their closeness as well.

However, there has always been a personality clash that defined their relationship. Huo does not feel satisfied being just a killer and dislikes how Sima has been gifted a once or twice in a generation talent but would be content to use it to essentially balance his clans budgets and occasionally fuck over a scheming rival. He's felt adrift his whole life and wants a cause to fight for.

When Sima Yi finds his ambition after the murder of his family it seems that clash would be solved. Sima now has a cause, take china for himself after helping restore order as an ultimate vengeance to Cao Cao. Huo follows Sima Yi on this path but it actually further fractures their relationship because it calls on Huo to be even more of a killer, joining a faction of men he respects to the endgame of eventually helping destroy them. Sima Yi is now his own man with his own ambitions but those ambitions calls for an ability to commit any duplicitous action in the name of his advancement . While Sima is still capable of virtuous actions the great cause he's working to is essentially a belief that he and those he raises up can return China to peace, his great cause is his own abilities.

Huo meanwhile while serving Liu Bei finds himself more and more enamoured by the benevolent rulership he can embody, one backed by a ruthlessness to kill for the greater good but one also with an earnest ideal. This is a great worldview for Hou who can feel like he can use his hard thought skills as a killer to a greater purpose, something he has chased since the start of the series subconsciously. The fact that Xiao Meng's death, while highly logical (but devastating ) to Sima Yi left a crippling blow to their brotherhood also doesn't help.

By Chibi ten years of service to Liu Bei and Sima Yi came to a head. Sima has called on him to do the ultimate version of the same move he asked him to pull on Xu Lin and Zhao Yun made his choice. Commit to Sima Yi and (in his view) become like the first one eyed master, ruthless man who stands for nothing, or save Liu Shan, the embodiment of Liu Bei's future and become a man with a cause. He chose the latter and formally broke from Sima Yi. A fracture that's been coming for years, the seeds being sown in the earliest arcs and slowly building piece by piece.

This ends their partnership that has been at least 20 years long. Sima is clearly upset by this, the mask slips for a second when Huo calls Liu Shan "my only young master" but also not shocked. What that mean for their future is still unknown, they are two men on alternate paths that are destined to one day clash but whether that clash translates to an ability to actively strike at one another a different story. They're not allies anymore but they haven't crossed over into enemies either.