lol, keep up!
this issue has been addressed so many times in all sorts of different threads here.
reread chapter 38.
well, if you really must know, highlight below to read.
*SPOILER!*
The original Kaname's soul is replaced with the soul of a Kuran ancestor by Rido, Kaname's and Yuuki's uncle (yes, therefore Shiki, Rido's son, is their cousin). The original Kaname's soul is considered dead. Because Rido was the one who revived him, Rido is now considered as the current Kaname's master and therefore cannot kill him.
*end spoiler*