[He trusted Yuuki in the past. In a way he still trusted her, but not enough to believe that she would ever place him before Kaname. Kaname's wants and desires would come before anything else. Even Zero's own well being.]
That girls own trust in Kuran is the downfall of anyone close to her. Her own way of viewing people is not the same as his at this time. She still sees people. He sees objects. Tools.Useful things. When they are no longer useful, they will be discarded.