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What I find so interesting about Jin Guangyao's explanation about why he finally decided to kill Jin Guangshan is that even though he's lying in his retelling to get the others to lower their guard, the original convo and the impression it left on him gives us such interesting insight into the Meng mother-son duo. This is what he says in Guanyin Temple:
“Why was a sect leader who spent money like water unwilling to do the smallest favor and buy my mother’s freedom? Simple—it was too much trouble. My mother waited for so many years, weaving together so many difficult circumstances when she talked to me, imagining for his sake so many hardships. And the real reason was only a single word: trouble. “This is what he said, ‘It’s especially women who’ve read some books who think they’re a level higher than other women. They’re the most troublesome, with so many demands and unrealistic thoughts. If I bought her freedom and took her back to Lanling, who knows how much fuss she’d make. It was best that I let her stay where she was just like that. With her conditions, she’d probably be popular for a few more years. She wouldn’t have to worry about her spendings for the rest of her life.’ “‘Son? Oh, forget it.’” Jin GuangYao’s memory was extraordinary. With such a word-by-word repetition, one could even imagine that drunk expression of Jin GuangShan’s when he said these words, “Brother, look, these three words are all that I’m worth to my father, ‘Oh, forget it.’ Hahahaha...”
—Chapt. 106: Hatred, exr
This is the actual scene and context of what Jin Guangyao is repeating:
Jin GuangYao had long since gotten used to this. He knew when he should appear and when he should not. He gestured towards Xue Yang and stopped in his tracks. Xue Yang clicked his tongue, his expression quite impatient. Just as he was about to go downstairs and wait, he suddenly heard Jin GuangShan’s gruff voice, “Women—shouldn’t it be enough as long as they water their flowers, powder their faces, and make themselves look as pretty as possible? Calligraphy? What a disappointment.” Those women all wanted to please Jin GuangShan originally. With these words, a flash of awkwardness passed over the pavilion. Jin GuangYao’s figure froze somewhat as well. Soon, someone giggled, “But I heard that back then in Yunmeng, there was a talented woman who charmed the entire world with her poems and songs—zither, chess, calligraphy, as well as painting!” It was clear Jin GuangShan was dead drunk. The wine could even be heard from his stammering voice. He mumbled, “That’s——not how things work. Now I’ve realized. Women shouldn’t play with those useless things. Women who’ve read some books always think they’re a level higher than the other women. They’re the most troublesome, with so many demands and unrealistic fancies.” ... Up on the pavilion, the women agreed with laughter. As though he remembered something from the past, he murmured to himself, “If I bought her freedom and took her back to Lanling, who knows how much fuss she would’ve made. If she stayed where she was, she might be popular for a few more years and she wouldn’t have to worry about her spendings for the rest of her life. Out of everything, just why did she have to bear a son, a son of a prostitute? What could she have hoped to...” A woman asked, “Sect Leader Jin, who are you talking about? What son?” Jin GuangShan’s voice drifted, “Son? Oh, forget it.”
—Chapt. 118: Villainous Friends Extra, exr
Jin Guangyao's scheming seems to be a trait learned from his mother. We've already seen and heard from multiple different characters by this point that Meng Shi bore a son in hopes that it would get her bought out of her brothel contract, but she did more than that. She learned the arts and education. She cultivated herself into appearing like any young woman from a noble family, even though she was a prostitute. The purpose of this crafted image was to attract the attention of a nobleman who would fall for her charms and hopefully free her from the brothel. The final part of that plan was to bear a rich man a son as, like one patron said, leaving a son to be raised in a brothel was both cruel to the son and embarrassing to the nobleman. And she wasn't aiming just to have her contract bought out, but to be bought out and her status elevated to that of a nobleman's wife, a plan that left her peers bitter. Unfortunately for Meng Shi, she picked the one lecher with a face thick enough to do exactly what the other patrons wouldn't. She bore a son, and Jin Guangshan disappeared like smoke. On top of that, her having a son decreased her popularity amongst other patrons. All of that hard work ruined in one fell swoop.
Jin Guangyao takes his mother's scheming and intensifies it. Instead of picking and sticking to one persona, he shapeshifts into soft, gentle, learned, efficient, helpless... whatever he needs to be in front of those he wants to curry favor with. However, he is also able and willing to do what his mother (willing or not) couldn't have: when those above him disrespect his station, he kills them. He forges a friendship with Lan Xichen by helping him escape the QishanWen and revealing curated moments of vulnerability with the other man to feign intimacy. He shows his efficiency and dedication to quality work to Nie Mingjue while subtly manipulating the man into attacking his enemies for him. He reveals his bloodthirstiness and petty, vindictive nature to Wen Ruohan, which earns him a spot as the clan leader's right hand man. And all the while, he is silently killing those who remind him of his low reputation, quelling dissent about his rise to power. But just like his mother, there's one target he cannot catch: his shameless father.
I won't make the argument that Meng Shi was wrong for attempting to use a child to manipulate her way into a marriage. The woman was enslaved to a brothel; there were no good means of escape in that system that didn't rely on manipulating some of the most immoral men in society. However, her lack of consideration (or possibly prioritization, since we do not get her actual thoughts) on how her actions would affect the child she schemed to have did backfire on her son. Meng Shi wanted her son to be what she thought his father would want: the powerful cultivating son of a cultivation clan leader. Jin Guangyao carries this same wish with him, that he be seen as his father's son. Instead, Jin Guangyao would be forever known not by who his father was but who his mother was: a prostitute.
What ultimately gets Jin Guangyao to commit to his father's death is not that Jin Guangshan disrespected his mother, but that he finally heard from the man's own mouth that everything he had been taught by his mother was a lie. It's not that he just hadn't found the correct persona that would make his father acknowledge him. It's that he would never be able to shapeshift his way into his father's acknowledgements. It's that no matter how many images he cultivated with how many different people, no matter how many people he killed in front of his father's face or behind his back, he would never be Jin Guangyao, proud son of the Jin Clan. Even to his own father, he could only be "the son of a prostitute" too uppity to realize that she'd never be a nobleman's wife and her son would never be a cultivator's heir. And that's why his father's death isn't the only product of overhearing this convo: Jin Guangyao's first order of business is actually to raze his mother's brothel to the ground along with all its patrons and prostitutes, already planning for the establishment of a Guanyin Temple with his mother's face in its place:
Jin GuangYao, “No, thanks. Save your energy, Young Master Xue. Will you be free the next few days?” Xue Yang, “Won’t I have to do it no matter what?” Jin GuangYao, “Go to Yunmeng for me and tidy up a place for me. Make it clean.”
If he were to be forever damned as his mother's son no matter how much he changed, then let her change for once. Let him be not the son of a prostitute but of a goddess, instead.