Hate.
Kazuki can't really say he's used to it, or at least he couldn't before the war with Festum. There had been only been one thing he's hated in life, and that is himself. He hates himself for hurting Soushi, for taking away his ability to pilot Fafner. He hates himself for never being able to apologize.
However, he becomes a veteran to hatred when the war arrives, he hates it with his entire being. Battle takes away not only his friends, but it takes away good, innocent people who didn't deserve such a thing.
He hates himself for not being able to prevent their deaths and being so damned helpless. He allows himself to hate war, as it is an entity, an idea, and nothing concrete. He honestly tries not to hate, but soon, he hates the creatures that started the war, the Festum.
He lets those feelings overtake him as he fights and hates himself for letting such an ugly emotion control him. More people are lost as the battles go on, and people are gained as well. But the hate progresses further in him, and he finds himself hating both things about people and the people themselves.
The way Soushi hides everything, the way he is so forgiving and grateful despite the pain that he's received. He hates Soushi; deep inside himself, he never forgives him for the first day he walked to school alone. He hates himself for holding such a worthless grudge.
And then there's Maya. He hates the way she looks at him. The way that her wistful smiles are of regret and love is disgusting. He hates her and the way she acts as though he's already gone and all she can do is watch him drift further away. He hates himself for giving her reason to act that way.
He hates the graveyard that he stands in, hates the way the name Minashirou Soushi reflects back into his eyes so clearly, as if mocking him. He hates how he can't stop crying whenever he sees it. He hates the current scene, the way Maya's hand feels on his back and how her voice sounds in his ears as she tries her best to calm him. He hates himself for not being able to save him in the first place.
He knows Soushi's alive out there, somewhere in the world.
He hates how much he doubts himself.