maybe he tried to say that kakashi was not careful enough to save rin may be he ment kakashi didnt kill rin on purpose may be an accident. we didnt see the fight before rin got killed right
or because Obito finds out that Kakashi had reason for doing it, but could have done something else to save her life. I bet this decision would come down to that comrades vs rules issues Kakashi and Obito have.