#I really cannot see it in a doomer way after how deeply I've engaged with this series

[i-heart-hxh]

Do you think Illumi’s comment about how Gon is just dazzling to Killua and a temporary fixation and Hisoka’s line about how transmitters are fickle and their treasures can turn to trash easily were foreshadowing for how Killua has come to see Gon? How at first Gon meant the world to him but now he’s basically done with him because of what happened during the CAA? Togashi could be planning to reunite them, but maybe this was his way of saying that Killua is done with Gon.

Hi! I actually see it in the opposite way to this view, almost.

As the audience I don't think we're supposed to agree with what Illumi is saying--that Killua doesn't deserve to have friends and that he'll ultimately betray them. It doesn't end up matching what happens in the series, nor does it match the overall message of what the series is about, and Illumi is an antagonist whose views the series repeatedly refutes (think: Alluka and how he sees her).

However, I also think that early in the series, Togashi was potentially trying to misdirect the audience with Killua's intentions. Killua early on was much more dangerous and edgy than he becomes as the series goes on, and there were a few pieces of foreshadowing that he might turn on Gon or the main group, which includes Hisoka's statement about Transmuters being fickle and their priorities changing rapidly, as well as Silva making him promise never to betray his friends and then expressing certainty that Killua will come back (as in, he was setting him up for failure because of the needle). There are also a few comparisons of Killua to Hisoka very early on that might fit into this (beyond the Transmuter thing), some even from Killua himself, and the scene with him getting frustrated with the ball game with Netero and then killing those guys as a result.

I believe Togashi laid this expectation down early in the series in order to subvert it and have Killua grow in the opposite direction. Killua felt like a "rival" type character or one who might eventually betray Gon/his friends, but instead he ends up being extraordinarily loyal to Gon (to a fault, even), he gets kinder and emotionally stronger as the series goes on, and he goes to extreme lengths to save Gon even after Gon pushed him away. He actively defies what his family expects of him! Even when he does go back home as Silva predicted, it's in order to break Alluka out of there and leave, and keep his promise to Silva by never betraying his friends--the opposite of what Silva actually wanted to happen when he made that promise with Killua.

Illumi's comment about Gon being "too dazzling for Killua's eyes" actually "comes back" in the form of the You are Light scene, where Killua uses similar language (in the original Japanese version) to describe why he can't look at Gon directly. That whole scene is about Killua comparing himself to Gon and thinking he's not worthy of him. However, Gon ends up falling into darkness himself and only Killua is able to save him--therefore, Killua becomes Gon's "light" right back. Because of what Illumi told Killua and the environment he grew up in, Killua himself didn't have faith in his own ability to be who Gon needed, but he managed it anyway because of how deeply he loves Gon.

He helps break Palm out of her loss of identity after she becomes a Chimera Ant, and becomes Ikalgo's light, too, by befriending him even though they're initially on opposite sides of the war.

We don't know the full reasons for Killua leaving Gon, however I suspect one of the reasons he leaves is in order to protect Gon from Illumi, who very well may pursue and endanger Killua and Alluka. Killua likely also needed a break from Gon after all they went through, but I don't think it'll be forever by any means. I made a post previously about why a reunion and reconciliation between them feels just about inevitable.

So, I absolutely do think those statements about Killua were "planted" intentionally within the series, but for the purpose of him breaking those assumptions rather than living up to them. One of the best things about Killua's character is how he initially seems so much different than he turns out to be. His commitment to and care for those he loves comes from a pure place in spite of the way he was raised, and he grows so much as a person throughout the series. He's a kind and courageous boy. I don't see him leaving Gon as a betrayal or throwing him away or legitimately "replacing" Gon with Alluka--they just came to a point where, for the time being, they had to take diverging paths. But I absolutely think their paths will converge again within the series, assuming Togashi can get that far.

I don't think Togashi is telling a depressing story where these tragic things happen between the two protagonists (who clearly adore each other) and then that's it, their relationship is over and they'll never get to mend the issues between them nor communicate all the things they haven't gotten to say to each other. There's a reason the separation is as brief and complicated as it is--it's intended to leave us with questions, and I believe those questions will be answered someday, assuming the manga is able to reach that point.