He never said "You betrayed me!" to Death. Walter said that to MATHIAS. Mathias made the pact with Walter, telling him of a warrior who was one of the best of all the land(Leon), and Walter wanted a fitting challenge. Mathias told Walter of Sara, and how to provoke Leon. But, Mathias knew that Leon would find a means DESTORY Walter, and on the point of death, Mathias would claim Walter's soul for himself, becoming the new vampire lord. Death was a servent to the one who held the Crimson Stone, hence was Mathias's servent. But, Death couldn't claim Walter's soul for Mathias UNTIL he was weakened to the point near death(because he was said to be an extremely powerful vampire).