I love Illumi Zoldyck, so thank you for asking about him! I really, really love him, and I’m always happy to talk about him. Killua, Alluka, and Illumi are my top three and honestly it cycles between them as for who is my very favorite. I love all the Zoldyck kids, actually. The adults can… I don’t with them. Lol.
Soooo I’m not sure exactly how it will play out. I would obviously love a redemption for him, though Illumi hasn’t exactly been obviously set up with a redemptive structure like, say, Shiragaki in BNHA or Mutsuki in TGre. But of the three major antagonists (Hisoka, Illumi, and Chrollo) he has been set up as by far the most likely to survive the ending and to get a measure of redemption imo. I think Togashi understands what’s behind him as a character: abuse, and a desperate grasping for freedom.
We see that in how Illumi and Killua foil each other, which Killua even acknowledges when he embraces Nanika and Alluka, telling Nanika he’s sorry for telling her to go away, telling her that he does not want to be like Illumi specifically. And we know what went into making Killua the way he is: his parents are controlling AF and abusive, subjecting all their kids to all measures of tortures.
Let’s consider the ages of the kids for a bit. Kalluto, Alluka, and Killua are all within like four years of each other. Then you have Milluki, at least five years older than Killua, and Illumi, at least five years older than Milluki. All of Illumi’s siblings would have had each other to help cope with the literal torture they were subjected to as part of their training.
Illumi would have had no one.
No one except his parents, Zeno, and Maha. Is it any wonder that of all the Zoldyck children, Illumi is by far the most robotic and obedient and seemingly the perfect assassin? Because the only way he would get attention or affection or praise or any form of family love was from his parents from the earliest of ages, and the only way for him to earn that was by being the perfect assassin to make his parents proud. But we know Illumi does occasionally disagree with his parents, and I’ll get to the Alluka situation in a bit.
Anyways. So Illumi is everything his parents trained him to be, and starting from the ages of 10-12 he was entrusted with raising Killua. Of course he treats Killua how he was treated, almost certainly. Of course he doesn’t understand Killua’s desire to have something else, to have friends and fulfillment through others besides his immediate family and specifically his immediate parental figure/mentor–which is Illumi, in Illumi’s mind. Because Illumi never had that.
Illumi is everything his parents trained him to be. And yet. And yet, that’s not enough for them. He’s the oldest son and he’s still not the heir because he’s somehow less talented than Killua. I don’t think Illumi is a-okay with this like he says he is. I mean, would you be? He’s done everything how they want, and it’s still not enough. But, he’s entrusted with training the heir. So he can still be the heir, in a sense, if the heir loves and trusts him, and obeys him. Illumi is too fragile a person to reject Kikyo and Silva, but he still steal back some measure of agency and desperately grasp freedom if he can control the heir (Killua). Illumi is not “uwu power for power’s sake.” Illumi’s actions to grasp power are the desperate graspings of someone who is being smothered.
Illumi prioritizes family first and foremost. But Silva says Alluka is not family, so Illumi says okay, then I can kill her because Alluka is a threat to his control of Killua and therefore to both his desire for freedom and his desire to be loved. And then he thinks it’s better that he can control Alluka and thereby Killua and thereby get it all–the brother he loves safe and with him forever, and control beyond his wildest dreams because of Nanika’s special abilities. It all stems from Silva and Kikyo, those terrible parents, and Illumi’s desire to be both the perfect son and to be free of them–though I don’t think Illumi even realizes he wants to be free of them in some ways.
The Election Arc really parallels Ging’s terrible parenting with Silva and Kikyo’s. Milluki is absolutely terrified of his parents and that’s clear in his protests and his clear understated asking for approval from his parents (in their conversations as they watch Killua and Alluka together in the vault). And then there’s the line about Silva being the one who asked Illumi to insert a needle into Killua. Silva is the one controlling the strings, controlling Illumi, controlling Killua thereby. The fact that Togashi said this outright does convince me that the ending won’t be like, Illumi taking the fall as the Bad Zoldyck and everyone else gets to recover.
Anyways, like I said, Illumi is Killua’s shadow. You don’t kill your shadow, you accept it and integrate/learn from it. Whether or not Illumi survives a final arc, should we get to that point in the story (and no Killua would never hurt him), I do think understanding between them is entirely likely.