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We shouldn't forget that Killua hasn't, ever, experienced the feeling of rejection from others until very recently. He only knew praise and care in the most sick and twisted ways: in order to emphasize how unique he is to the Zoldyck's bloodline, he got hurt and tortured. +
This is a reasonable way, for him ofc, to show that he's unique and valuable: getting hurt. That's why, in the dodgeball match, getting hurt and praised for his sacrifice made him feel special, and he was for Gon, of course! But unlike his family, Gon got hurt with him. +
What gave it so much more meaning, he chose to get hurt in order to help Gon, and Gon emphasized that Killua the only one for him, and accepted the sacrifice. And stayed to sacrifice too, by his side. It was still getting hurt, but not by torture. +
Makes sense then that Killua feels rejected when prevented from fighting by Gon's side. If getting hurt is the only way he was ever taught to give and receive love, if Gon does not let him this time, is he protecting him or rejecting his act of love?+
Of course, Gon wasn't trying to reject his friendship and love, but for Killua, makes sense. Getting hurt equals to get praise, and Gon praised him in the most purest way. So, that's when the shinjuu idea came: the ultimate act of hurting himself.
He only knows pain, either for good or bad purposes. He can't measure, yet, how being casted way from fighting wasn't opposing to his love. Dying is the ultimate sacrifice. The pain that becomes eternal. And for his measures, the greatest act of love.
It would mean, eternal and unconditional love. He spent his whole life being close to death, but overcoming it by being strong and unique. Now, the same need to feel loved and special makes him want to give up on the strength of overcoming death.+
It's a powerful - and dangerous - way to show how much he loves Gon. But waking up to other ways to show and receive love is something that he's learned fast. The shinjuu is not the only option. He didn't want Gon to get hurt and still loved him, why does he feel rejected?+
He gave up on the shinjuu wish, where he was already accepting Gon would 100% die and he would follow, and decided to fight for their lives. Overcoming death together, not to be put under pain again like with his family, but to give them a second chance to fix themselves.
They can't make their lives torture in order to prove how reliable, strong and caring they are. It's the same old cycle over again. They can't hope the other will get hurt for them anymore. There's where the hospital scene kicks in and lead them to the separation scene. <3
That's why the separation is more optimistic than sad, in my opinion. It's overcoming death to be happy in life, not to end up in the same cycle of sacrificing again. It's okay to love and to feel you're capable of doing that, but to live and to learn is the best way of loving.
He wasn't rejected when he got prevented to get hurt. Getting hurt should NOT be your #1 act of service. But for Killua, he had to learn that in a chaotic situation - it wasn't about him personally, it was Gon rejecting himself. He was still, the most loved one.
Now, we should be the ones put to think about how much, sometimes, we express the wish that things never changed between them. We are the ones afraid of change. We should root for them to live as healthily as they can.
