On why Izuna's death was the turning point in Madara's life:
bit of meta bit of headcanon anyway lets get into it
I don't think Madara's descent into madness was caused directly by Izuna's death, but more like it was the pivotal moment of no return. Bad End Route locked sort of thing.
I think what happened after Izuna's death was that Madara completely shut his heart to others, he shut down and withdrew inwards and his actions became more like lashing out than leadership. At that point, Madara was essentially incapable of forming any new bonds.
Soon after, his bonds with his clan could no longer withstand the strain, followed by his final loss to Hashirama where he just.... gave up basically..
Hashirama tells him that (old friendship and sentiment aside) if he kills the head of the Uchiha they will seek vengeance to which Madara responds that no one will avenge him.
whether he is stating fact or projecting onto his clansmen doesn't really matter at this point, because reality is that at that point Madara was hurting, defeated, and in a most absolute sense, alone.
Madara gives up because there are no longer any bonds tying him to this world. For one who has lived his entire life as a weapon, warrior, and protector of the Uchiha, he is now quite literally without purpose.
and then Hashirama brings up the village again, in complete seriousness. he was so serious in fact that he was 100% totally willing to kill himself over it when Madara asks (which he did rhetorically, kinda like "yeah maybe when pigs fly", but hashi in his earnestness was like omg that's totally doable and fair i'm gonna do it! to everyone's absolute horror)
suddenly in the absolute tarpit of rock bottom he was in, Madara finds that there was still one bond going strong underneath the years of war and battle, and so, once more, he gets up, and tries again. he drags himself back up, enters an unfavorable alliance, and Konoha is formed. for the first couple of years he is kind of recovering again, very slowly putting himself back together.
but then his bond with Hashirama is put under strain and - in Madara's eyes - is unable to withstand it. Hashirama's choices felt like rejection to him. His acquiescing to Tobirama felt like betrayal.
because Hashirama saw the village as above all, and Madara saw Hashirama as his final salvation, and these two perceptions proved fundamentally at odds with each other because Madara became incapable of forming new bonds after Izuna's death. He was incapable of seeing the village as his, he was incapable of putting it above all and integrating into it like Hashirama wished he would.
add in a nice sprinkle of the fear invoking reputation and Madara's generally abrasive/intimidating demeanor, it meant that no one was willing to try. Hashirama was the only one, and yet he-
Madara sees his final bond fray, and becomes completely disillusioned. his obsession with the tablet was a desperate search for answers, an explanation, anything that would tell him why he was fated to a life of strife and suffering. why everyone was fated to repeat the same senseless losses over and over and over.
if we did this earlier Izuna would be alive and he wouldn't have had to die and maybe my other brothers would be too and maybe Hashirama would have his brothers and maybe all the funeral pyres would have never had to be lit and what did any of this ever achieve? what did we win? why did Izuna have to die? is this mess of a village what he died for? i can see it fall apart like it will happen tomorrow why are we doomed to repeat the same mistakes? why did Izuna have to die? what did i spend my entire life fighting for? what did i spill all this blood for?
why did Izuna have to die?
he finds a new purpose, one that can only be fulfilled by someone like him, who is free of worldly bonds, who is powerful enough and tenacious enough and angry enough to rewrite the world.
so he cuts off the last fraying string tying him down.












