Originally Posted by Fenrii

His parents were killed for no other reason than they were in a location where war was being waged.

He was starving to death and the dog was the first friend he ever had. It died because, like his parents, it was in the proximity of a war zone.

If the flashbacks continue I bet we see Yahiko also become a casualty of war.

I think Nagato doesn't so much just hate the leaf, he hates War in general. If he, as one person can become some evil and so destructive that everyone else quits fighting each other out of their fear for him, then he accomplishes his goal even if he has to become what he loathes in the process.

I bet that if we get even more information we discover that the reason each of those bodies was someone J-man knew from before is because they were the greatest "weapon" of each of the warring nations. Nagato took their weapons away from them and then re-made them into his own. When he finds out about the tailed beasts and how they are sealed into the Jinchuriki to make the ultimate weapons he upgrades his plans and decides that he needs those instead. If he has an example of each nations weapon, then crafts the ultimate weapon from what are the most powerful beings in existance he can end war. Sure... lots of innocent people die now, but if in the end he succeeds and stops war how many more will he have saved?

It is a twisted sense of justice and a really messed up view of the world, but through these flashbacks I can see why he is so messed up. All he has ever known is Pain and loss because of war, so he will stop war at any cost by making the people who wage it know what it feels like to be on the losing side. It is boring, but it does give you an idea of why he is a sociopath.