Heaven and Earth
Kenshin is faced with one last major obstacle to his peaceful life: the return of Yukishiro Enishi. Driven mad by his sister's death at the hands of Kenshin, Enishi is out for revenge. He doesn't want to kill Kenshin -- not directly. He wants to see Kenshin suffer.
Enishi's revenge takes a very cruel form. Knowing that doing anything to Kenshin himself would be meaningless to a man who had already suffered so much, he decided to take away what Kenshin loved most: Kaoru. In pretending to kill Kaoru and planting a fake corpse that fooled even Megumi, Enishi succeeded where no other opponent of Kenshin's had. He made Kenshin lose hope, give up, and wait to die. The saddest things he had ever said were in volume 24, chapter 207, page 24. Upon seeing Kaoru's apparently lifeless body, he fell to his knees and threw away the Hiten Mitsurugi Ryuu, threw away Himura Kenshin himself, and saw himself as nothing but failures upon failures -- all his speeches, his promises, and his ideals were reduced to nothing, and he turned his back upon the world and left it by disappearing into the Rakuninmura -- a den for the outcast and fallen -- and sinking into an impressive demonstration of catatonic depression. Even so, he still took his sword with him. Though he had chained it up in its sheath, he still refused to let it go.
While in Rakuninmura, all his friends and companions tried their best to persuade him to return. Misao, Sano, and Yahiko did their best and failed: talking of revenge, responsibility, facing Enishi again. Kenshin didn't budge. Finally, Tsubame braved the town and its rowdy inhabitants to plead for Kenshin to save Yahiko's life during a fight...and succeeded. An old man he had met in Rakuninmura (whom Kenshin never recognized as actually Tomoe's father) also helped Kenshin realize that he wasn't yet ready to give up on the world: evidenced by how fiercely he still clung to his sword. Saving people was not merely something Kenshin felt he had to do to atone. It wasn't even something he loved to do. It was his life, his truth, as the manga put it. He simply can't help himself. Someone (I think it was Sano) mentioned that although Kenshin the swordsman was incredibly strong, his heart was also incredibly soft. And although he already knew it, and had demonstrated it by refusing to kill even people such as Aoshi, Shishio, and Enishi's henchmen, it was only during the final volumes that he fully realized that all lives were important -- even those of his foes. Even his own. What had been a hazy, childish ideal finally crystallized into clear and absolute truth (for him, anyway). This reinforced his oath not to kill which, when added to the will to live he found during the Kyoto arc, made him well nigh invincible in terms of sheer willpower.
This was the final (and most important) turning point in Kenshin's life. Here, he found all the answers he had been seeking, and embraced the life of one who will always protect and never kill so fully that even Saitou had to accept it. In fact, the moment Saitou realized this, he stopped referring to him as "Battousai" and called him "Kenshin" for the first time.