#7. Ch. 6, "Just Too Creepy!" Reason to smile: being Gittarackur

We simply must start this off with Gittarackur, the king of genuine and approachable smiles. This is our second Illumi panel, and he spends it with a smile creepy enough to dissuade our resident rookie-crusher. We'll see Gittarackur's entire face change soon, with his jaw filling out and his drool disappearing, but this smile sets the tone for Gittarackur. Why would Illumi choose such a smiley disguise? There's no wrong answers here. (It's a shame he didn't get to try the poisoned juice, though.)

#6. Ch. 326, Hisoka's Toybox. Reason to smile: being imagined as a toy

I've docked a couple of points here because this is an imaginary Illumi, but I've added a couple of points for being phenomenally cute, so it still comes out very well. In a sea of toys unhappy to be in Hisoka's toybox, Illumi stands out as the happiest and most lively toy around.

#5. Ch. 326, "Ready, Set, Go!" Reason to smile: driving a truck into his little brother

Do I understand what this panel is doing? Yes. We've got Illumi fading into the dark background, flattening him out and also making him look like Nanika. It's also a clever way to communicate the scene while minimizing drawing time, given Togashi's health issues. Do I like this panel? No, I think it's jarring and its style gets in the way of its point. BUT! Do I like this smile? I do. He looks so genuinely delighted to be on this mission against Killua. He just loves to be hunting down his family members. It's the most fun he's had in years. Remember: if you have an unusually deadpan assassin, he might just be lacking in sibling-hunting enrichment.

#4. Ch. 36, "No need for us to fight, eh?" Reason to smile: getting his way

The only thing better than fighting with your brother is not fighting with your brother. This is his smallest smile, but one of his most expressive. Look at those eyebrows! He's so happy! Killua's obeying him, he's about to get his Hunter license, and all is right in the world!

#3. Ch. 336, "That's my theory." Reason to smile: theorizing

Illumi treats us to some long theorizing and exposition about Nanika, then turns to Killua and absolutely beams at him (and the audience) while asking if he's right. How much of this did Illumi actually say to Killua? No idea. How much of it is correct? No idea. But Hunter x Hunter characters love nothing as much as they love exposition, and Illumi is a master of the craft. I'm glad he's proud of himself, especially since he's about to get teleported away.

#2. Ch. 333, "Nanika... is MINE!!" Reason to smile: feeling possessive of his incredibly powerful sister

Illumi has a tendency to count his eggs before they've hatched. He's absolutely thrilled at Nanika's power. Last panel, he said "I want this power... above all else!" Now, he's saying that this power is his. Not quite, Illumi, but I'm glad you're excited. Also, check out that smile. He looks like a cat just absolutely drowning in catnip.

#1. Ch. 336, "I could turn Killua into a complete puppet!" Reason to smile: could turn Killua into a complete puppet

I don't think he particularly cares about political power or even wealth except inasmuch as they permit him to do his work. (It is, however, much more convenient to be rich!)

Nanika can provide many things that pure politics can't, and Illumi has no idea what the limits of her power are. Maybe she can raise the dead. Maybe she can make Killua wantto come back home. I think he wants access to that power on its own merits, but I also think he's curious and wants to probe exactly what the limits are. (He kept sacrificing butlers to her back in the day!)

With Nanika in particular, his choices are to kill her, to control her, or to let her loose. Letting her loose not only risks Killua using her power and thus sacrificing himself and the Zoldyck family to pay the price, but also risks Nanika trying to take revenge. (I'm not sure Illumi is cognizant enough to recognize that risk - he certainly doesn't seem to fear that Killua will do the same - but given that he views Nanika as a monster, I think it's reasonable that he might.) Once the choices are kill vs control, controlling obviously gives access to this tremendous power while killing destroys it. He's willing to kill her, as evidenced by his direct statements to Killua and his commands to Hisoka, but why not use a power if you can?

Once he's set his mind to controlling her, killing her feels like a failure.

Even if he's not aware that Killua doesn't love him, he miiiight be getting the sense that killing Alluka would make Killua unhappy with him.

Fundamentally, Illumi wants enough power to 1) do his work and 2) protect his family. Having access to Nanika means he's got options for dealing with any imaginable threat to the family, including, possibly, internal threats like "Killua doesn't want to be here."

On a broader level - Hunter x Hunter spends a lot of time thinking about what it means to want stuff. To steal a phrase from Media Club Plus, "a Hunter is someone who has a pathological aversion to ever feeling content," and Illumi gets to stand on a shared pedestal with Hisoka as representing the worst outputs of the Hunter worldview. (Notably, he and Killua are the only Hunters of the Zoldycks, and they're both characterized by tremendous desire.) He wants to control Nanika because he wants control on a deep, fundamental level. He gets his kicks from controlling powerful people, like Hisoka gets his kicks from destroying them and Chrollo from stealing from them. His entire being is structured around controlling people; the more powerful they are, the more powerful he gets to feel in turn.

I saw your tags (on this post) wondering if he's trying to control his parents as well. I don't really think so, because I think he's in a pretty satisfactory give-and-take relationship with them - they give him lots of latitude to pursue his projects - but I do think he's planning to rule the next generation (read: Killua) in a much stricter way than his parents ruled him. I think this whole project is more about future-proofing the Zoldycks than it is about direct, current benefit.