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"Rin was my only light", some just refuse to see what he meant by that. Remember, this was a war, shinobi was dying left and right. I don't think Obito even had any parents. Outside Kakashi and Rin, did he even have any friends? He was only 13. I can't really blame a kid for attaching onto something they love during such turbulent times, looking for that "ray of light". When Rin died, he had nothing. The wars and corrupt governments only made it worse, plus, there's Madara TNJ.
To put it simply, no matter how dark the world was, he wouldn't have started a war if Rin never died.
But, if Rin died, and the world was a peaceful place full of aboveboard and gentle people,
then he would have never started the war.
Even Kakashi admitted the world was hell, and had doubts of his own. However, they're two different people. Obito also had Madara whispering in his ear, and Kakashi's comrades probably supported him.