Shishio is already dead. He came from the Palace of Earth Spirits and got welcomed by Satori after Okuu found him. She basically told him that he was in paradise now. But he was probably disappointed since he wanted to beat THE HIGHEST JUDGE OF PARADISE and take over hell itself. Huh, he's pretty gutsy!
Right around the end of the Kyoto Arc after Shishio spontaneously combusts and Houji commits suicide by slicing his neck he finds himself in hell with Shishio and Yumi. Shishio finds hell to be suitable for types like him so he decides to take it over. Because Hell is full of beings like Shishio it'd be easy as hell to take over. And Houji being fanatic about Shishio joins him without question.