While the current representation of zombies is of a brain eating living dead person wandering the earth to destroy humanity, they have been with us for hundreds of years.
Within the history of the New World, the Western Hemisphere, zombies are more tied to the religion of Voodoo. The mythical zombie is a reanimated dead person under the control of a Voodoo Priest or Priestess who has the Power. The zombie is at command of the person of Power.
This iteration of the zombie is not that of a brain eater.
While there have been recorded cases of zombies, it has been determined that the zombie was never actually dead but had been drugged with a powerful natural psychotropic drug that left the victim in a malleable hypnotic state easily controllable.
With today film makers and fantasy story tellers, tales of zombie apocalypse make for entertaining stories.
I don't think it is an excuse to kill things that look human, because it is easy to establish aliens attacking and there is no reason for them to not look human. They are just capitalizing on an existing genre that offers an opportunity to be gross and have fun. Look at how many dress up as zombies for Halloween.