In the early Meiji era, after participating in the Bakumatsu war as the assassin "Hitokiri Battousai", Himura Kenshin wanders the countryside of Japan offering protection and aid to those in need as atonement for the murders he once committed.
In the story Kenshin is said to revert to the murderer Hitokiri Battousai he once used to be. Why would killing revert his full personality back in to that of a murderer permanently?
NOTE: He did sometimes temporarily revert to his murderous self but soon after gets pulled out of it by people such as Kamiya Kaoru.