Kaneda always has been special.
He was a gang's boss, rode an awesome motorcycle and dated any girl he did want. School was for nerd and stupid people; his scores didn't make him appear a bit less special. Really, for what purpose math and physics existed anyway?
So; Kaneda was just like Tetsuo ever wanted to be.
When Tetsuo first met him they were still very young, but Kaneda retrieved his toy and he looked so tall, that Tetsuo wished to be with him forever. Because Kaneda was just like he wanted to be, also because Kaneda was brave and strong enough to had fight with older boys just for him. He was so happy when Kaneda saved him all the time… until Tetsuo grew up, and Kaneda started to treat him like a child, even thought they were about the same age; by that moment, everything collapsed.
The only thing Tetsuo ever wanted was to be just like Kaneda. He wanted to be acknowledged by Kaneda: to be able to save him, instead of the other way around. Tetsuo wanted to be stronger more than anything, but Kaneda never allowed him and Tetsuo started to hate him because of it. He did never forgive him.
So, when Tetsuo gained power and indeed became stronger than Kaneda, he hated him even more when he didn't get any recognition at all. When Kaneda still wanted to save him (just like when they were six years old again; his favorite toy stole again by some unknown boy), Tetsuo allowed to blow out all his hatred towards Kaneda and then, he decided that if Kaneda didn't want to acknowledges him, he'll kill him.
Even when, in reality, Tetsuo loved him and the only thing he ever wanted was to be just like him.