I just had an epiphany (not really, I am just sleep-deprived, I have a headache and as usual I miss KilluGon so this combination led to thoughts).
It has come to my attention that during the palace invasion there are 4 characters who pinpoint that Killua looks sad (because of Gon pushing him away). I never thought much about it besides the fact that Togashi made other characters to emphasize Killua's pain to show us how much Killua cares for Gon and as a result how hurt he is to be pushed away.
But then it came to me that each character that points out Killua's sadness represents a stage or an aspect of Killua's emotions that are related to Gon.
The first one to notice that there is something wrong with Killua is Shoot.
When Shoot was first introduced he seemed to share a trait with Killua. They were both very cautious because they were afraid of a stronger adversary which caused them to be overly cautious.
Shoot (not his whole character, but that common trait he shares with Killua) seems to mirror Killua before he removed the needle. When he was afraid he would lose Gon due to his family's infuence on him.
The second individual to take notice of Killua's emotions thoughout the war was Ikalgo.
Ikalgo basically voiced out what is part of the reason that Killua seems sad. However, Ikalgo at the same time is the enemy that Killua spared and befriended, resulting in Killua being Ikalgo's light that gave him a chance to start over a new leaf, representing that way Gon and Killua's dynamic where Gon became Killua's light and offered him the chance to start anew.
Ikalgo (again, not as a whole character but that certain aspect of him) mirrors Gon's positive effect on Killua and how Killua came to admire and care for Gon.
The third person that remarked that there was something going on with Killua was Meleoron.
Meleoron was told by Killua himself that Killua is prepared for a double suicice which made Meleoron question things. Things that he decided not to voice.
He simply decides to keep things to himself because Killua is either unable to tell or does not want to, which reminds me of this panel.
This panel perfectly sums up the way Killua does not have the courage to voice his feelings so he chooses to hide what he truly feels.
Meleoron's decision not to ask him mirrors Killua's trait of keeping to himself.
The final individual that was aware of Killua's pain was Palm who did not actually make an internal remark, but was told by Killua himself about his struggles.
Killua's emotions bottling up finally caused for his feelings to explode during his battle with her, resulting in him letting his real feelings out in the open to which Palm responded that Killua is actually the person Gon needs the most.
When Palm was first introduced her plot was treated like a gag that could be said it was probably meant to highlight Killua's feelings by making him protective (and jealous).
Palm's statement could not only represent Killua's need for reassurance, but also Killua's need to come in touch with his emotions openly.
Think about it, Killua fights an enemy whose feelings and memories are separated by a wall, which is removed by Killua admitting to his own struggles openly. This could serve as an allegory for Killua to overcome his own wall that stops him from expressing himself.
Finally, I find it interesting that 3 out of the 4 individuals are people that Killua fought with, except for Meleoron who joined the team through Gon.
If I take it a step further, one could say that Killua's battle with each of these people who were aware of his sadness represent a step closer to his development.
Killua removed the needle after being defeated by Shoot and deciding to protect Gon before leaving him -> Killua eventually overcame his fears and escaped his family's emotional abuse.
Killua spared Ikalgo because of his loyalty -> Killua showcases his own loyalty and good nature that was able to come to the surface thanks to the time he spent with Gon.
Killua fought Palm to whom he almost lost until he brought her back to her senses by being honest -> Killua needed to come in touch with his emotions and admit to them openly.
Killua did not fight Meleoron who decided not to ask him any questions -> Killua still needs to learn how to voice the questions he wanted so badly answers to from Gon.
So in the end there is one battle that needs to be conquered and that is communication.
Thank you for coming to my ted talk. Maybe this really does not mean anything and I am just overthinking either because I used to pay too much attention in literature class back in the day, or because I am too obsessed with KilluGon.