Koichi lived because Kira got arrogant, his anger took over and beat the shit out of Koichi when he could've obliterated him in an instant. It's Kira's own fault Koichi and Jotaro are still alive and it would make no sence for Koichi to get killed of now because he just had character development. Just in my opinion but this is Koichi best arc and Kira's worst simply because it shows Koichi's development and Kira acted out of character because he is usually cool and collected.
(Sorry for the long comment but i just thought i should explain why Koichi wasn't killed)

This is all happening because of that damn sock, Koichi needs to die.