Well basically, After learning Kakashi and rin were attacked by hidden mist shinobi, eventually, rin dies by mist ninjas, obito blames it on kakashi, which is why now, he hates him, but moreover, he sees that it were shinobi of the mist, who ultimately killed Rin...
So in time he would repay his vengeance to them, by controlling the mizukage, and started the bloody mist era, having the mist killing each other, to be worthy or something, my theory.