It's from Bing-ge's perspective (the OG ML of the novel world). He resents his Shizun (the OG one, I feel like there maybe some behind the scenes story for the OG Shizun in the novel world to act as a villain) and continuously tortures him. There's this Soul Summoning Array of sorts that allows Bing-ge to bring the Shizun's soul back each time he commits su*cide due to the repeated tortures. Gradually his soul weakens due to repeatedly being called back up. The day when the OG ML visits Bing-mei's world (ML), he resents his luck to have MC as his Shizun. He returns, only to witness the scene of his Shizun piercing his heart with a jade pin. He tries to call back his soul but realises it has already broken. After repeated attempts, he uses some sort of complicated technique to get his soul back at the cost of half of his life. But due to being already broke, when the soul gets placed in the body it doesn't have any 'senses'. He is unable speak, smile, cry or have any kind of reactions to his surroundings. Bing-ge realises that he has 'lost' his Shizun forever and would never be able to get him back. He makes his Shizun his cultivation partner and dismisses his entire harem.

• OG Shizun has a very sad background story which has been mentioned in the OG novel. He used to wander the streets like ML, but had the sect leader guy as his companion, so was not all that lonely). He got caught by traffickers and sold to a rich family. They treated him well for appearances sake but was repeatedly abused in reality. YQ (the sect leader guy) did visit him but was unable to notice anything amiss with SJ (the OG Shizun).
• Since YQ left, SJ had to take up all the torture by himself. He (SJ) somehow found an opportunity to escape and burnt all the people at the house to death.
• He aimed for the peak leader position of a quite mountain (due to his own withdrawn personality) and could be considered as a prodigy in his own right.
• He accepted OG ML (Bing-ge-I use this word particularly to differentiate between the two Bing-he's-the ML and the Novel ML) but hated his guts because of the fact that ML, too, was in a similar situation as that of SJ. Despite being the same, OG ML was able to get to cultivate at the optimal time whereas SJ had to start cultivating much later in life. Thus, reaching a bottleneck in his cultivation and unable to advance any further. This offsets his jealousy and negative attitude towards the ML. (Well, I say negative but he did commit those 'crimes' against ML which ultimately twisted ML's personality).
• Now, onto the current timeline. OG ML tortured SJ a lot (the acts were too gruesome), killed YQ (the current sect leader) and repeatedly called back SJ thus, destroying his soul.

Now, onto the fact that SJ might not be that bad of a guy. Well, his backstory is pretty sad but that doesn't justify his actions. During his discussion with YQ (he was dying at that point because of OG ML's repeated torture), he acts as if he doesn't repent but in the end, he does understand that it was him to blame for his and his sect's (ML destroyed the sect) current tragedy/plight and to some extent does feel repentant. So, according to me, though he is a bad guy, but maybe it was just the circumstances that forced him to become so.

BTW, forgot to mention this, but the current MC is Shen Qingqiu who is paired with Bing-mei whereas the OG Shizun who used to 'torture' the gong is Shen Jiu. He's paired with Bing-ge (the OG stallion ML with a 300+ harem). The one-shot is based off on Shen Jiu X Bing-ge.

P.S: If you thought just because I wrote so passionately it might be a continuation of the OG but no it's not. It's a fan-fic. I really believed initially it was an extra released at a later date. But after confirming it on the spoiler thread with another member, I came to know it's not. But it's a really good one. Enjoyed it.