"Hahaha..Well, do you five Kages think you can do any better?
Surely you should all have realised by now...
There is no such thing as hope!"
"The concept of hope is nothing more than giving up. A word that holds no true meaning"
He probably realised that even if he would become Hokage it wouldnt change a thing, there still would be hatred and war around. And after experiencing the pain of loosing someone you love firsthand he did not want it to happen to anyone else.
He came to the conclusion that becoming Hokage wouldnt let him archieve his goal, thats were Zetsu takes a big role. The real Madara entrusted Zetsu with the task to find someone to carry on his mission (and revive him later), Zetsu found Obito dying, saved him and explained him that Madara also wanted to create a pure world without hatred and wars (either that or Zetsu just lied in that regard to manipulate Obito and keep Madaras own goals in the dark).
Tobi/Obito liked that plan and agreed, he probably realized quick that he would need (alot) more power to archieve his goal so he decided to kill Rin to awaken his MS (and his S/T techniques).
These drastic decisions would explain the (massive) change in his personality if you ask me.
I wouldnt be suprised if Kishi would pull a Itachi again in the end. Making Tobi/Obito a good boy. Maybe Obitos true intention isnt to cast Tsukuyomi on the world. Maybe he just wanted to create a big enough enemy (himself) to force the different Shinobi Countrys to work together and stop the hatred that way?
He would have archieved his goal already if that is true.
Maybe thats why he was shocked after hearing that the real Madara got resurrected, he knows that the real Madara doesnt want peace and will be very hard to handle?
The first picture he is LATE as usual. Let's just wait for the next week chapter, a lot of things will be explained. I think Rin got killed because of Kakashi, he said ''he failed to protect her'' and he would join them (that is when he died but got resurrected by Nagato)