After the humiliating defeat of his Warrior John was truly at his breaking point. He lost his only way to rid Pandora of the bandits that took so much from him not only that he didn't manage to avange his daughter that the Vault Hunters savagely killed the only thing he truly cared about.
How could he the hero of this world loose to a bounch of child killing bandits. He knew this was his end, he knew this ever since Lilith punched the relic into his face. His only salvation was a half assed plan that might preserve at least something from him, his soul.
He didn't believe in Professor Nakayama and his stupid Ai preservation plan after all even if he was successful they would't be him. So after years of research on sirens and their powers, mostly thanks to his observation on his daughter he found the solution with enough Eridium he could do it he could punch a hole big enough through the fabric of the universe that his soul could slip away.
So when Lilith finished him of he was ready and he knew it would work after all in the end. The hero always wins.