I'm not sure it's fair to put all that blame on Snyder though. You gotta think that a lot of the action was written into the script with Snyder just following that. Now this is merely an assumption on my part, it could very well be Snyder's doing...
You're right, it was in the script. Goyer confirmed it in this interview he did right here.
Look at the 5:24 mark.
But Snyder certainly played a part in the depiction of it, listen to Snyder's hilarious justification of the massive death toll.
Zack Snyder on mass destruction wrote:
I wanted the movie to have a mythological feeling. In ancient mythology, mass deaths are used to symbolize disasters. In other countries like Greece and Japan, myths were recounted through the generations, partly to answer unanswerable questions about death and violence. In America, we don’t have that legacy of ancient mythology. Superman (who first appeared in ‘Action Comics’ in 1938) is probably the closest we get. It’s a way of recounting the myth.”