Why would Himura Kenshin turn back into Hitokiri Battousai? 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.
In the OAV series Rurouni Kenshin: Trust &amp; Betrayal, we get to learn more about Kenshin and his trip from being a simple orphan to Japan's most feared assassin Hittokiri Battousai, and then to the peaceful Ruroni. 

As Hittokiri Battousai, he wasn't exactly evil. He had good intentions, and saw assassination as the only way he could help the weak and the downtrodden. Through his interactions with Tomoe, and her final fate, Kenshin begins to realize that he had been doing it all wrong. Helping people by spilling blood was not the way to go. That's when he dons the character of pacifist Ruroni Kenshin, who refuses to kill no matter the provocation or how evil the other person is. 

But that conviction is a very hard one to keep, especially for a trained assassin such as Kenshin. He knows that if he slips just once, and kills as a means to an end, he would fall back in to the endless cycle of blood letting that dominated his life as Hitokiri Battousai.