The Night of the Living Dead was initially about breaking stereotypical thoughts. Romero is heavy on political and social commentary in his movies. It gets a bit tiresome.
However, the popularity of zombie movies corresponds with the general feeling of dislike of the government and the feeling that some people are taking advantage of the system. The zombies represent that masses too politically dead to notice that they are being controlled. The hero is the one who is fighting to wake up the people not completely under control of exterior influences. As the fight shows, it is hard to keep people on the right path and they end up getting taken over by the masses. And, the sad story in the end, the hero is taken out by those too zealous to bring things back to the status quo.
As with all monster movies, there is a cycle. When people need a hero to get them out of their daily dull drums, hero movies become popular. We've been in "We need a hero" mode for way too long, I believe.
RE: Romero's Death - A look into Zombies