Gah... after going slowly and putting a lot of effort into the start of the flashbacks, Kishi then speeds through a lot of the major stuff and rushes the character development. It's a lot like how the start of the Obito flashbacks were so slow, and then suddenly the next 3-4 chapters felt like too much happened too fast, with little explanation.
It'd be nice to hear more than just "After that, Madara started to change" after spending so much time seeing the kid Madara. And I'd have liked to see Madara awaken his Mangekyou / Hashirama awaken his Mokuton (the Mokuton is particularly important, seeing as he's still the only Senju with it that we know of).
Madara's descent into darkness would have been cool, but since it all happened in this chapter with little explanation, it seems kinda contrived (and making MADARA's evil seem contrived is pretty hard to do).
Cool that Tobirama killed Izuna... it's symbolic, but it's also clear that Madara will have an extreme grudge against Tobirama, and will only reinforce Tobirama's distrust of the Uchiha... he probably sees Madara in every shadow. I wonder if Tobirama feels guilty about killing Izuna, after seeing Madara's level of hatred? Maybe he can see, just a little, why Hashirama's way of doing things would have been better?
And Tobirama used "Hiraishingiri" to deliver the finishing blow on Izuna!! Incoming speculation on the nature of Tobirama's space/time jutsu. Perhaps he teleports his sword strike forward, making it an almost unavoidable strike?
Damn, you wrapped up my sentiments exactly. Thanks bro lol!