Maybe spoiler:

As far as I know, NMJ at one point referred to him as a prostitute's son, but other than this, I can't for the life of me remember if there was anything else, srry

Guangyao is a narcisistic person... He had to be the center of everything, ALWAYS... He went as far as to kill his own son and wife(she was even his sister, just shows how psychopath he was) just to be the talk of the town...
He'll do anything to anyone who say a "no" to him... The only reason he didn't kill Lan Xichen, is because Lan Xichen never said "no" to him... He didn't appear to be someone who would hinder Guangyao's plans...
For me... Guangyao is far worse than Xue Yang... He's a true psychopath, that kind of psychopath that seems like the kindest and most honest person, but hides a creepy killing intent within...

From my reading of the novel, it was absolutely NMJ's comment about JGY being son of a prostitute that put him automatically on JGY's kill-list. The moment anyone refers to JGY as the son of a prostitute (insulting JGY's mother, who he loves fiercely), JGY is already plotting how to take them down.

NMJ almost killed him twice, and almost sent him to his death by getting him to confess. JGY did try his best to get on NMJ's good side again without luck. NMJ's condition was for JGY to get him XY's head (which is an asshole move, conditioning for someone to be nice for you). After NMJ kicked JGY down the stairs and called him son of a whore, it was JGY's last strand of being nice to him. Those are the two buttons you DON'T push on JGY knowing his history and NMJ went there.

I'm not saying he deserved his death, but he should have at the very least expected it. NMJ's whole sense of justice is too flawed. Justice without empathy. These chapters in the novel really changed my views on both of the characters, one of the better and one for the worse.

JGY would have killed him anyway... Because NMJ wouldn't let him become the sect leader knowing his crimes... As I said, JGY will kill anyone who says "no" to him or try to stop him... That's why Huaisang had to act as an idiot, so JGY wouldn't suspect him...
Remember the way JGY killed NMJ? He already started the plan even before NMJ said that to him and kicked him downstairs...

Remember that JGY killed an ENTIRE sect just to cover the fact that he killed his own son... :v

But he had been disrespecting and disobeying NMJ long before he got called 'son of a prostitute' by him. And i don't see how and where JGY tried his best to 'genuinely' get on NMJ's good books. He had always put an act of being good in front of everyone including NMJ and LXC even after his crimes were exposed. I just think that NMJ deserved to be respected the same way he respected LXC. Cuz NMJ is practically JGY's savior. Without his kindness would JGY really be in that position? The way i see it, JGY killed NMJ when he started becoming a threat completely ignoring his favours. JGY is selfish. He sees no one but himself :/

Yes, thats cuz NMJ never did anything wrong other that calling him 'son of prostitute'. And that too, i think he just blurted out of anger and didn't mean it.

And what do you mean by his sense of justice is too flawed. I wont say its perfect or anything but Are you implying that xue yang doesnt deserve to be sentenced to death after what he did? *-*

1. JGY was never aiming to become a sect leader.
2. JGY was trying his best to get with NMJ's good graces because he cared about him. Like Su She, he values people who show him kindness. What NMJ did when he was still with Nie sect, was something he remembers hence why he tried so hard to get on his good graces.
3. JGY's pattern is killing those who attacked him first or by following his fathers orders.
4. NHS didn't act as an idiot to fool JGY. He played an idiot since Cloud Recesses so he wouldn't have to be the leader and wield the saber.
5. JGY added the extra piece of the song AFTER he was kicked. Clearly said in the novel, and clearly differently done in the tv series.
6. Go read the freaking novel and don't fact check me with CQL

He never disrespected NMJ. What he did from killing the Jin commander up until killing Wen Ruohan was something he had to do to follow his father's order, he had to put an act to show that he was a Wen follower. After that, he never disrespected NMJ, well, until he killed him but NMJ already did his deed then.

In NMJ's empathy chapters you can see JGY trying to get close to him again. He offered him alcohol, women, arts and other things but NMJ was never interested in anything but training and killing the Wens. Even learning Song of Clarity to help him while LXC couldnt. That was JGY's way of getting closer to him. NMJ's condition of them becoming buddies again was XY's head.
LXC is the one who got him into Sunshot after JGY saved him. NMJ defended him from the Nie soldiers insulting him. He tried to chop him after killing WRH. Kicked him down the stairs. Insulted him. I'm sorry, I don't see how and why JGY would hold NMJ in the same light as LXC. I freaking wouldn't. I would have dropped NMJ's bones on the first attempted on my life, kudos to JGY.

JGY isn't all a facade. No one can hold it that long, one is putting an act to the public and another is spending it with someone you care about. He will never be MY again for the change he went but he is who he is. Plus we are looking at WWX's pov, the smiley faced JGY is what JGY shows to the public.
You, like the cultivation world, are focusing on just the bad and flashing those things out. Same happened to WWX, and it's a theme in this novel that nothing is black and white. Not WWX, not JGY, not XY. What of the watch towers? LWJ and JGY were known to be the only ones to help the common people. JGY saved Qin Su at Langya as a Wen soldier. JGY had one of the best positions at WRH's side, and he chose the side where our 'protagonists' are. A lot of his actions were plain horrible and he's seen them for what they were, but selfish isn't the word I would use. I would put naive or foolish. His case is another good lesson MXTX dropped, don't follow your parents wishes blindly.

He has righteousness in mind but no empathy. He can not see past the name Wen (particularly Wen Qing's case), he can not get on someone else's level, believes that the system is just for everyone when it clearly isn't. He grew up with privilege vs the lowest of low (JGY). Even if he does say words of encouragement, saying is easier than doing so. His view of the world is very black & white, no shades of gray.
As for XY. This is NMJ's CONDITION for him to stop being an ass to JGY even after becoming sworn brothers. XY may deserve death, but he is not in JGY's hands, but JGS's. In a prettend situation that JGY did give him XY's head, JGS wouldn't have forgiven this and JGY knows this.
More on NMJ and his righteousness can be read from here. A bit more insight if you will https://hamliet.tumblr.com/post/187738379679/nie-mingjue-righteousness-without-empathy

Idk if this was just in The Untamed or in both the drama and the novel but instead of playing The Song of Clarity properly didn't he play something that eventually led to Chifeng-zun's Qi deviation?

He did. But the big difference between canon and tv show is that JGY adds an additional piece AFTER he gets kicked. It's where he stops and turns against NMJ, and it's a big key moment. In the tv show he was out to get everyone, rather they attacked him first or not. Similar how he planned Jin Zixuan's death in the tv show while in canon it was an accident (MXTX said this)