It never struck me in the past, but Kenshin was putting away at half a brain all, "Yare yare, gozaru," trying to recover from the PTSD of killing numerous people. He is trying to live this new life, and become the new person.

When the past is dug up, be has a deep personality change. When Saito attacks, we see the real? old?Kenshin buried under all the layers of peace. Although we have seen the cracks in the past, I think this is the first time in the series that he seriously considered killing someone and thoroughly became his old self.

I don't know if it is intended, but Saitou's first name is Hajime, or beginning. The character is supposedly based on a real person with the same name who served as the captain of the third unit of the Shinsengumi and later as a police officer.