it isnt Zacks fault. He could have left Cloud to be a test tube project forever. He didnt,he saved him. Zack was the last thing Cloud saw as "Cloud" and so took on the personality of his best, and only true, friend.

It is Shinras fault, i believe. Shinra, for letting Hojo work there. Hojo for creating Sephiroth. Sephiroth for bring Zack along. Zack for taking Cloud. Cloud for hidding from Tifa. Tifa for never telling Cloud when she saw him finnaly. Sephiroth again for thinking Jenova his real mother, and Hojo for never telling his son he was the true father of Sephiroth. Jenova for having been Sephiroths madness,Sephiroth for burning the town. Cloud for not dragging Zack away while he had the chance.

Getting my point? One thing leads to another. Everything inter linked. You cant really blame one person