A lot of the time, the "wall to wall condemnations" of a certain movie or play as anti-jewish (or anti-black, anti-christian, etc.) is not so much due to the movie's content, but because of inter-organizational rivalry. No jewish organization wants to be the only one NOT to condemn what OTHER jewish organizations already labelled "antisemitic", for fear or seeming "soft" on antisemitism or "ignoring" it. So there is a snowball effect: once one organization doesn't like it, they all don't.
I haven't seen the movie, of course, since it is still a screenplay. It COULD be that Mel Gibson is a christian zealot and an antisemite--just because he is a celebrity who usually plays sympathetic characters and heroes doesn't mean he's one. This seems unlikely, since so far as I know he was never on record saying anything remotely anti-jewish. However, there is an alternative explanation: it's just difficult to make a movie of Jesus' life that makes the jews look good--for the same reason it's hard to make a movie of Moses' life without disappointing Pharoh's fans.