Makoto Shishio was a legendary murderer in the Meiji era, and one of the most reknowned swordsmen in the world. He was described as being Kenshin's shadow after he stopped killing.
Shishio's new body
He went on to become what they considered the second Battousai, and killed people just as Kenshin had, albeit more ruthlessness involved. Eventually, however, Makoto was considered to big of a threat for the Shinsengumi, and was eradicated from existance...or so they'd been lead to beleive. Shooting him in between the eyes, and then after he was paralyzed burning his entire body to near nothing, to the point where he was no longer even identifiable, they left him for dead.
However, it wasn't enough, as Makoto Shishio lived on, and eventually killed the men who had tried to end his life that way. He stopped killing, but continued to scheme...and ten years later, in the Meiji Era, that scheme would finally go into action.
Kenshin was asked to kill Shishio, some eleven years later, despite the government knowing of his vow never to kill anyone.
Kenshin Himura, Battousai no longer
Shishio, however, was fine thinking about Himura trying to kill him, as he had complete confidence in his ability to defeat what he considered a fallen hitokiri. He had survived the burns, albeit being forced to wear bandages around his entire body to keep his body heat at minimum. This experience with fire had lead him to see things in a whole new light, and lead him to begin using fire as an example for lots of things that he never connected it with before. Even his sword style revolved around fire, with the help of friction. He believed the world he lived in was indeed hell, and the only way to survive it was to become a demon filled with madness.