There's a theory I have. Refering to how can Obito have fought Minato as a grown man. If he was supposed to be 14 at the time.
Sometime after being crushed, he unlocked the Mangekyou Sharingan, unlocking his ability to use Kamui. As we know the MS can only be unlocked by the emotion of losing a best friend, which leads me to believe his selfless act of saving Kakashi played a role in the activation as he didn't want to lose him. This caused his MS to be activated and he be transported to his dimension. In this Dimension having no clear control of the technique he ended up spending years there gaining power and control of the technique. Of course his supposed years in side that Space/Time Dimension weren't actual years in the real world but that dimension (Kishi has a fondness for DBZ and this dimension in my opionon has some sort connection to the Hyperbolic Time Chamber
which Gohan and Goku trained in. A day in the real world = one year in the chamber, thus Obito being stuck in this chamber grew stronger as he worked to master his new found power. He grew and had multiple "years" to dwell on the idea of war.
However this is the part where I'm trying to link when he began to gain Madara's ideals, however there has been been evidence in the Manga that states the two are already acquainted and working together.
559 page 2
Madara comments on Nagato - a person born way after Madara's time. How does a dead person know this information if someone hasn't told it to him? Very odd unless someone (Obito) had a direct line to him.
560 page 1-2
Madara looks on as the shinobi try to figure out who the Masked Man actually is and Madara comments on it. Stating things obviously aren't going as planned for him.
561 page 9
Madara speaks about THIER plans with Kabuto.
The thing I can't figure out is how the two ever became linked in the first place. How did Obito get involve with Madara at all? Why is Madara on Obito's side? How did Obito survive the crushing all together w/o being healed? (which I have another theory for but no real proof)
Either way after gaining those ideals, he went out and fought against Minato and yadayadayada... did all that stuff... which leads us to the present.
and there you have it.