It is not and never will be Zach Snyder's work, though. It is, plain and simple, Alan Moore's. Moore's right is the fact that it's his work that's being put up on the screen. He has every right to criticize.
That he's criticizing before so much as seeing the movie or bothering to see it is over-dramatic, but it doesn't change the fact that if anyone has a right to piss on the film, it's the writer of the original work.
And let's face it - Hollywood has already urinated on a number of his works. The League of Extraordinary Gentlemen is most prominent in this. Other films, while not necessarily bad, were rather not the comics - From Hell, the film? Decent. But it significantly warps and twists the work it's based on. The movie goer who has read the comic may not care as much, but it's not their work that's getting redone. The personal attachment there is going to be considerable.
Comparison's to Christopher Tolkein fall flat; he's not someone living off the corpse of his parent. He's the originator of the work.
And, gee, yeah, Alan Moore is so worthy of hate, just because he gets into a righteous fury over Hollywood trampling over his work. A guy getting pissed because people put words in his mouth that he never uttered (such as claiming he supported certain films when he didn't) or significantly change what he's written is all too worthy of hate. Right up there with, oh...child molesting.
Yeah, he's a little over the top in his disdain. But he has the right.