Writing Wu Yanzi is difficult because there's so little of him in canon, and then he also sucks really badly, so it's tempting to make him a very flat scum villain.
I try not to write flat villains always, but with SVSSS fanfiction, it feels like a particularly offensive crime. And it seems like an especially "missed the point" direction to take when writing Shen Jiu's POV.
On one hand, Shen Jiu is one of the least generous narrators that I've ever written. He's cruel, vindictive, violent, hateful, jealous, envious, deeply traumatized, and prone to assuming the worst about the world and everyone around him.
On the other hand, Shen Jiu is perhaps the most likely to be generous towards Wu Yanzi, even when he hates his master. They would have spent a LOT of time together. Traveling. Eating meals. Sleeping in the same locations. Finding work and then executing the job. Wu Yanzi was probably all kinds of abusive, but if Shen Jiu felt like he was in legitimate danger constantly, he probably wouldn't have stayed so long. Abusive relationships are rarely 100% awful all of the time, which often makes victims doubt themselves. And Wu Yanzi apparently did teach him some things and let him keep a weapon. He fed and sheltered Shen Jiu. He took Shen Jiu away from the Qiu manor fire, saving him from any authorities instead of abandoning him.
When Shen Jiu was apparently in trouble with Yue Qingyuan at the Immortal Alliance Conference, Wu Yanzi does reveal himself and intervene, instead of just slinking away, and we don't KNOW that Wu Yanzi was purely motivated by hatred towards Cang Qiong and the cultivation world, or self-preservation / hiding their activities. Wu Yanzi also could have been motivated by some genuine desire to protect his student as well! We just don't know!
Personally, I think Wu Yanzi was very likely an extremely bitter, vengeful man, who both resented and enjoyed his criminal life. For one thing, killing and mugging people at the Immortal Alliance Conference seems like a pretty stupid plan to me. I think he probably dragged Shen Jiu through hell and took his anger out on the convenient target of his student frequently.
But it's important to remember that Wu Yanzi must have a backstory. A personal history that explains his inability or unwillingness to be a kind teacher, to regulate his own pain, to temper his own self-destructive impulses. There must be reasons why he has skills but no allies, no family, no friends, no sect. He could have been decent to Shen Jiu sometimes, in his own way, as well as awful.
People can often be extremely cruel towards those who remind them of their own flaws and insecurities. This may have been Wu Yanzi to Shen Jiu, but also Shen Jiu to Wu Yanzi. Shen Jiu's feelings on Wu Yanzi have to be very complicated! (Even if I think Shen Jiu has repressed most of them!) Like Shen Jiu himself, Wu Yanzi was still a person, even if he was also a scum villain.
