lol well obviously this is personal opinion but the idea is i once saw FF7 as a master piece. for the longest time i regarded it as not only the best FF game(and i have played most of them) but also as one of the best video games of all time. and to an extent it still is. however with the creation of advent children a few things happened. yes it was visually spectacular but for the most part i didn't like the way the characters were portrayed. originally cloud was upset at aerith's death but not so much that he was crippled by it. however be that as it may, advent children was also the catalysis for the surge of FF7 sequels, prequels and everything in between. by expanding on the FF7 universe far more than any other final fantasy it lost a lot of its mystery which is part of what makes FF7 a great game.
then we come to zack. zack(as well as vincent) were originally minor characters. yes zack's back story is entirely relevant to clouds but on his own as a character he didn't serve much purpose seeing as he is dead before the main plot of the game even begins. in fact one of the most interesting parts of the game is uncovering the truth about his death(this is optional) now not only have we seen his death they have completely fleshed out an entire back story for him. now he has lost all of his mystery.
so zack( along with vincent ) to me represent the industry milking the game for all its worth. on several different forums including this one (as well as monty oum's dead fantasy page) i always see people go "they should put zack in this, they should put zack in that". two examples of this was dissidia and dead fantasy. in both cloud is already involved, and yet they want zack anyways (despite their fighting style being almost exactly the same) and overall i just feel like people have lost sight of the original games but also have completely forgotten the other FF games which were all good in their own right.
its petty i know, i just wish FF7 would go back to what it was but of course thats impossible.
and that is why i hate zack for what he represents.(long enough answer? XD)