As anyone reading this probably knows, Illumi is the first child to the Zoldyck family. The hxh wiki lists him as 24 at the start of the series, which gives him quite the gap on his siblings: when he’s 10, Milluki is 5, and Killua doesn’t exist yet. When he’s 12, Milluki is 7, and Killua is just born. All of that is very important - it means that Illumi spent a long time as his parents’ only child. And being his parents’ only child (and later their eldest by such a big gap) could not have left him without significant issues later as an adult.
Early on in the series, during Silva’s talk with Killua, Silva says that he raised Killua as he’d been raised himself. It’s a short enough jump to assume that Illumi received the same training, and then another to assume that for years, Illumi had been considered as his family’s heir.
And again, it’s quite a small jump away to assume that his childhood was a lot worse than Killua’s, even though they both grew up with the very same expectations - because in Illumi’s case, there was no one to take the fall if he failed (while Illumi is Killua’s backup). (For reasons I won’t go into in this post, Milluki was probably never considered heir material.)
So: Illumi grew up bearing a lot of pressure and expectations, and unlike Killua, he wasn’t strong enough to retain his own personality, and broke under the pressure. That made him unacceptable as an heir, later on, since Silva and Zeno aren’t looking for a puppet incapable of thinking for itself.
(I’d like to digress a little and talk about Illumi’s nen. Illumi is a manipulator who manipulates people, i.e. turning them into his own puppets (ironic, isn’t it? A puppet’s puppet! Togashi quite likes doing this sort of thing with nen-typing; just look at the chains Kurapika chose to manifest, practically holding himself prisoner to his past). At the start of HxH, Illumi is the only Zoldyck child who knows how to use nen (except for Alluka), and I can’t help but wonder why? When? How? Was his nen ‘forced’ out of him the way Gon and Killua learnt it, except by someone as ambitious and malicious as his family? Was this what finally broke him? It’s implied that nobody ever bothered to teach nen to Milluki - perhaps it’s because of how badly Illumi reacted to it. IDK, it’s just a headcanon, but it makes an awful amount of sense.)
After Killua is born, things change. For whatever reason - perhaps Silva and Kikyou were too busy with Alluka and Kalluto, perhaps another reason - Illumi is put in charge of Killua’s care and training: for all intents and purposes, he becomes Killua’s third parent. It’s possible he even replaced one of their parents as parent-figure entirely (my bet is on this one, with the replaced parent being Kikyou). All of a sudden, he found himself responsible for a helpless boy’s chance at survival.
This scenario is called ‘parentification’, and is a state where a child takes over their parents’ roles. Parentification can be extremely damaging to a person’s psyche, leading at adulthood to problems relating to others, anger issues, overprotectiveness, isolation, depression, anxiety, control issues and/or excessive obedience, a disconnection from themself, self-esteem issues, and the list just goes on and on. Illumi exhibits a staggering amount of these symptoms.
Illumi is one of HxH’s main antagonists, and for a good reason. He is not a good person. He does not want to be a good person. But that doesn’t make him any less damaged by his uprginging, a damage that he passed on quite enthusiastically to Killua, who’d ended up usurping him as the family’s heir. I wonder, though - does Illumi regret losing that title? Or does he feel relieved to have it off his shoulders?
[ Illumi // Milluki // Killua // Alluka // Kalluto ]
What I’d really like to see in fic is asexual!Gon/Killua where Killua confesses his love for Gon as “more than just a friend”, and Gon is all confused because what he confused as “bffhood” is actually him being in love with Killua, except not attracted to him. And Gon is really…
He’s just mad because he can’t acquire all the apple juice that I’m acquiring. (x)