Less Xuan Su as a malicious hungry parasite, and more Xuan Su as an unwilling perpetrator of harm.
Cause like, one one hand, we've got Xin Mo. The sword characterized as a metatextual but also literal example of 心魔, a heart demon, that staple of wuxia/xianxia as the literal and metaphorical manifestations of one's fears and obsessions. It's constantly hostile and emotional, so endlessly hungry for suffering and violence that it'll rip open into new worlds to find it. It makes sense that only Bing-ge can tame it, because he also wants those things when he finds it and has the power to give them.
So then what does a narrative foil of all that look like?
Well, Xuan Su's name for one thing, 玄肃. Seven Seas translates it out to 'dark and solemn' but it can be stretched a bit further.
We've got Xuan, which can be 'black' or 'dark', which explains why one of only two descriptions we get of it is as an "all-black longsword" in vol 2. But also it can be 'mysterious', 'abstruse', or 'profound', extending from its character's original etymology of emptiness. Really suits its meta as a plothole point for PIDW, re: how often it got name-dropped by Airplane but never once made an actual appearance. Then follows Su, which similarly can have a couple of meanings depending on the usage. Primarily it's 'respectful', 'solemn', but you've also got 'to clean up' or even 'purge'.
All in all, it really give off the vibe of a righteous sword in a lot of aspects, which makes sense given that Yue Qingyuan says he was going for 人剑合一, or the path of Unity of Man and Sword as it usually shows up in wuxia/xianxia novels. There, it's typically the penultimate skill of swordmanship where sword and wielder are now so aligned in purpose and connection that their true skill can be reached, often with a lot of focus on the swordsman in question having to make peace with themselves and their heart demons.
Just like how YQY and SJ's relationship mirrors LBH and SY's, it's fun to interpret Xuan Su as maybe more the opposite of Xin Mo's hunger instead of as a variation of it. If Xuan Su is as cool-minded and calm as the name implies, it's hard to believe it finds any joy in destroying the life of the person who chose it. Maybe it even could have loved him, or maybe even loves him still, hurt by how the bond that should've perfected them both will instead eventually kill the boy who chose it.
It's tragic as a headcanon, really. They could have been the best partners for each other, perfectly balanced, made for each other.
But of course, that's the tragic irony, right? That YQY failed in unity with Xuan Su purely because of one thing: his heart, and how preoccupied all the love in it already was with a promise to someone else.
