Pain.
It wasn't physical. His body had gone numb long ago. When that boulder came crashing down, half his body had already been reduced to pulp. This pain was in his heart, as though countless blunt knives were churning inside it. Rin falling, Kakashi's blood-soaked hand, the shocked look Minato-sensei hadn't had time to hide... they replayed before his eyes over and over, crueler than any jutsu, flaying his soul.
Darkness. Cold, clinging darkness. Was this the world after death? No, wait... could I still think? Still hate?
Then a voice-or rather, a will-slithered in like a venomous snake. It called itself Madara Uchiha. It said he wasn't meant to die, that this world was wrong, that there was a plan to start everything over: the Eye of the Moon. To create a dream world without loss or pain.
He believed it. Or rather, he desperately needed something to believe in. Hatred and despair were the richest soil, and Madara's words were his only lifeline. He became "Madara Uchiha," wore the Spiral Mask, concealed his shattered heart, and advanced the so-called "plan" from the shadows. Watching Konoha, watching his former comrades struggle amid pain and suspicion, he occasionally felt a flicker of twisted satisfaction. See? This world really is that ugly.
Madara died. The old bastard had schemed his whole life, but in the end, he still breathed his last. He left behind the snide Black Zetsu, supposedly a manifestation of his will, to continue assisting-and watching-Obito.
He kept playing "Madara," carrying out the plan. Collecting the tailed beasts, reviving the Ten-Tails. But deep inside, some corner of his heart kept sneering. Create a false dream? How is that any different from dying? Rin... would a fake Rin still be Rin?
Until that one time, deep within the Demonic Statue