It is because izuna the loss of izuna drove him to doing what he is doing now.
Izuna doesn't even make the top 25 reasons Madara is doing this.
he cared more about Hashirama than Izuna..
If Madara can "care" then 100% dead on. If Madara cared about anything/anyone it would be in this order:
1.) Hashirama
2.) Proving to the world and/or Narutoverse History that he truly "beat Hashramai" in their final battle. Obito always says something along these lines with "History has it wrong who won at the Valley of the End," and/or something always having to do with History will be re-written, in essence, as Madara being Victorious in the end.
3.) Konoha (I will explain this below)
No... Several things that happened in the past
His hate/rage/jealousy is easy to trigger
Like when Hashi became Hokage instead of him... Hate started to grow.. Develop..
..The...
...Descent...
Into... Darkness...
Getting a little deep here but I can agree with this. If Madara becomes Hokage maybe this never happens. If I had to add something into this even before the Hate starting, I would say Madara became envious first. Here is a man in Hashirama who not only created Konoha and the Peace System that is still intact today, but he was able to stop the Senju and Uchiha fighting, have them SETTLE in One-Village together, and convince Madara's own clansmen/blood that he should be Hokage.
Actually, I think 5 emotions/characteristics came into fruition together:
Fear, Hatred, Envy, Jealousy, Respect. Whenever they first battled, Madara most likely realized (or probably was distraught/shocked that someone stronger existed) Hashi could kill him. In the end, Madara had realized (not to mention came up with the longest strategy in existence) he will ALWAYS be #2 as long as Hashi is around no matter what he did based off the Uchiha/So6P Tablet such as taking his brother's eyes, etc... leading to the eventual battle that lead, as well as put everything/his grand scheme into fruition which was Hashi's DNA. We know the rest.
I think it's a mix of things. I'm starting to see Madara as more of a Joker from Batman type of character. He is just doing this for his own amusement and just happens to be pissed off.
I had written a theory along these lines in another thread and I obviously couldn't agree more with you. It's like Michael Caine/Alfred puts it to Bruce Wayne when he tells him that story of the Ruby-Stealer. Some people don't do it for money, power, etc...they have no reasons, they just want to watch the world burn.
Though everything written above would suggest I believe Madara is doing this for the title of WINNER/BEST THERE EVER IS, WAS AND WILL BE, I believe AT THIS POINT IN TIME based off of what Kishi has shown us about Madara (as well as the dialogue that Kishi has written for Madara), I think Madara is past all of that and once he realized he could be the 5-Kage with less than 5% effort (Madara knows all too well what Kage means - THE BEST), why does he even need the Moon's Eye Plan and/or put everyone in a Genjutsu? We know all that crap he told Obito, Nagato, etc...is just "Madara's Guide to Manipulating through Fake Idealism" and at this point the idea of winners/losers, strongest/weakest, fair/unfair and so on, means NOTHING. Madara doesn't need a Genjutsu anymore because he is and can choose to be any of those things. He is the WINNER, but can be the LOSER if he wants too. He is the Strongest, but he can choose to be weak and so on and so forth. If Madara wanted to put the entire
Allied Shinobi Force under a Genjutsu which would have them join the battle on his side and/or trick them into killing themselves, etc... he would. Instead he is, for lack of a better term(s), purposely and not purposely playing mind games with everyone, trying to summon Kurama, pretending Obito still fits into his plan, and so on.
Because I could go on with this forever, I will put it in layman's terms: Madara could keep sending that Comet Jutsu of his over and over on the other major battlefields until the only people left are Naruto + Gang and Madara + Obito. He most likely hasn't done it because he has already used the Jutsu and knows how devastating it is. As Obito said he is a "Kid/Child" and to give my answer as to why Madara did this/is doing this would be multiple reasons since it has changed with time. At one point, he was motivated to control the world through a Genjutsu PROBABLY because he thought someone could stand up to him AKA there would always be a Hashi around, no matter the time period. But now, seeing how someone like Obito can go around as Tobi while claiming to be Uchiha Madara and having Madara see this and know this first-hand as in his mind already changed History, having won not only the Battle, but the War.