Well, the truth hasn't yet been revealed in the manga; is Walter a traitor tot he Hellsing Organization or a victim of the Millennium Organization? Certain aspects throughout the story support both theories, but i want to know what other hellsing enthusiasts think.
I think he was brainwashed (he even says to Integra "you truly are the master i once so cherished!"), but on the other hand I want to know what the Colonel meant by "it was decided 50 years ago that you [walter] would work for us". Also, Alucard calls him a traitor, inferring that he made a conscious choice. WTF. I'll be so pissed if he turns out to be a backstabbing bastard.