I hate zombies. I'm a science fiction and fantasy writer, so it's in my job description to think about these things. I don't mind them in a fantasy setting - I particularly like the draugr of Skyrim...but when the concept tried to enter the realm of science fiction...meh.
First of all, the zombies of the zombie apocalypse are frightfully boring. Always a product of a strange new disease, an engineered virus, always behaving in the same goddamned way. In groups, stumbling around, trying to kill everything that moves. They do work effectively as a metaphor for the consuming, dominating nature of contemporary American society, but that's a one-shot thing. Attack on Titan uses a similar metaphor for the giants who consume humanity, only, notice that there are no knockoffs to that series. It works once. After that, just another gimmick.
They'd be so much more interesting in a science fiction apocalypse story if they were mutated into something more sensible. The head-crab zombies of Half-Life 2 are the perfect riff off of the concept. They make zombies comprehensible in the Half-Life universe. Excellent storytelling. If you haven't played Half-Life 2, you need to get the game and experience Gordon Freeman's journey. I'm such a fan, guess who's in my home-screen background image on my phone: the G-Man.
Imitation is flattery. I chose to emulate Marc Laidlaw and the other developers at Valve in my exploration of the zombie apocalypse, the short story "Retirement Age." You can buy it here: http://www.amazon.com/dp/B00OXX9TRU Much more interesting to think about the technology that could create classic zombies, who would use it, and the purposes to which they would put it.
First, there is only one technology that could hold the "promise" of creating undead creatures, nanotechnology. Diseases, even artificial ones, have a patient zero. Modern medicine understands the spread of disease. There's an entire branch of medicine focused just on this one thing, it's so important to the health of humanity. Epidemiology. Look at the smackdown the epidemiologists of the CDC - yeah, there's an entire federal agency devoted to this one thing - gave to SARS, are currently giving to Ebola, because they understand how biological entities cause disease. They understand the patterns. Blood-borne diseases find it especially hard to spread. We didn't all die of AIDS and rabies, did we? Rabies is a ferocious disease, by the way. Spread by contact with blood and body fluids, in its end stage it causes infected mammals, including humans, to become raging beasts bent on attacking and biting anything that moves. Kinda like zombies. Has the world fallen to hordes of rabies monsters? Nope. "See that man over there, foaming at the mouth and trying to bite everyone? Shoot him." Also, rabies victims loose the ability to eat or even drink, their bodies wear out after a few days, their brains eaten up from the inside, they die...for real. Nothing in nature can prevent a dead body from rotting away. If there ever were a zombie outbreak, their bodies would be eaten by dogs and pecked away into nothing by birds, rotted into the soil by bacteria. You gotta bend molecules, reshape the human body to keep it moving after death. You need micromachines.
We can't yet build spooky shit like that. If someone could, they couldn't be from the Earth. ET hates us. But why not bombard the Earth with asteroids? That'd kill us real good. The only thing is, our nice, juicy biosphere, the global ecosystem within which we live, that's unique. Even if life were everywhere in the galaxy, none of it would look like life on Earth. You're aliens, you hate us because we're a bunch of kill-crazy cave men with nukes and not a lick of sense, but you love the dolphins. You deeply crave the scent of flowers. How to smoke us without destroying the Earth's beautiful, abundant, glorious life? Asteroids out, nukes out, violence is out. We can fight back. Even engineered diseases are out, since we have just enough medical technology to defeat them.
What you need to get the job done is an army of robots programmed to kill only humans, preferably made up of humans. I suppose you could use your gnarly mind-powers to create a kill-crazy religion...only, maybe they've tried that already, didn't work out. Martin Luther was an alien? Mohammad was an alien killbot? Hong Xiuguan a telepathic ET with a grudge? What next? Nanotech. They build nanomachines, distribute them throughout the environment, wait until they reach saturation in the human population, and activate them all at once. There will be freaks with powerful immune systems who can withstand the nanomachines, so convert the infected into killing machines to hunt down the immune. Wait a hundred years - seriously, if you can build sophisticated nanomachines, you can engineer yourself to be immortal - come back, pristine Earth, beautiful biosphere intact, no more killer apes.
Isn't that just a little bit more interesting than an unexplained rampage of dead people? Not counting the draugr, of course. I <3 draugr.