In this instant, Tetsuo committed murder. When he killed Nobuto, he was doing it to protect his daughter. It was a spur-of-the-moment crime of passion, with the only premeditation being picking up the killing weapon. However, Tetsuo knows Kyoichi will die as a result of his actions. Tetsuo had to plan this in a way that couldn't be detected. Tetsuo knows that unlike Nobuto, Kyoichi has potential to be a better person, to rise above their current station and not be a scum of society.
Tetsuo knows all this... and goes through with this anyways. THAT'S what makes this situation so incredible.