dam, i wonder what kanames secret is, and his plan started when he saw the twins, im starting to think that maybe he had a hand in the death of zeros parents and that he wanted zero to meet yuuki and become her protector, i always knew he was using zero but i never thought he was using him for so long, maybe he planned to kill all the purebloods from the beginning but because he cant kill rido he had to figure out a way to get someone who is powerful enough to kill him, he did that by getting ichirou to be devoured by zero to gain all the powers he hadn't claimed which then allowed zero to transform the bloody rose and kill rido, but after that yuuki chose kaname and wanted to leave with him, which stopped him from going out and killing the rest of the purebloods
i think what sara said to zero is "the man who allowed your parents death was kaname, he has been using you since the beginning"
so zero now has a personal reason kill kaname other than his duty to the hunters, he wants to kill kaname for manipulating him, and allowed him to turn into a vampire and allowed the death of his brother, well thats just my theory, i always seem to perceive kaname as the main villain :s