Vash is trying Rem to keep alive in his soul. For Vash, Rem is like a mentor, more than that, she is the person that makes him continue his life. But, when Vash took someone's life, breaking the teaches received from Rem, his world falls apart. In this moment happens the most beautiful scene from the anime: Vash discovers that he teached someone else Rem's values in life and this gives his the power to fight against his greatest fears.
