Since I am (I assume) the only religion scholar currently contributing to this blog, I feel that it is time for me to speak on the religious aspects of the undead. (I know you are all expecting an edged weapons post from me, but in keeping with this month's theme, I have opted for something more philosophical - deal with it.)
This is a very muddy topic from a theological perspective. For the sake of simplicity we will be talking about the Abrahamic faiths (Judaism, Christianity, and Islam), and we will be handling doctrine in only the broadest sense - so don't post a bunch of comments that point out the dozens of exceptions to the sweeping generalizations I'll be making. We all know there are about a million different sects with about a billion different viewpoints; I am just trying to hit the high spots.
There are several questions that spring to mind immediately:
1) Do zombies have souls?
2) Is it ok to kill them?
3) Where do their souls go?
On first blush, it is tempting to answer these questions with, "No, yes, and who cares?" but I realize that many of our readers will find that unsatisfactory, so here are my considered answers.
1) I am tempted to say no, because transition to the zombie state is generally preceded by death. At the time of death, the soul is believed to leave the body, and so any subsequent action of the body would be in a soul-less state. This, however, overlooks the recovered zombie - the zombie who is freed from his curse and returns to normal upon some external event. An excellent example would be the South Park zombies - upon the death of the main zombie, the other zombies (those not cut into chunks earlier in the episode) were returned to normal. To propose that their souls were temporarily absent and then returned to them seems to fly in the face of what is accepted about the soul, as well as posing difficult questions about where the soul was during this time; concepts like limbo or purgatory are just too crazy to speculate about.
These zombie-like species which aren't actually dead really do pose a problem. The zombies Clay mentions here are a good example; another would be the Reavers in the Serenity 'verse. None of these creatures is actually dead per se, so presumably they still have souls. It's easiest to view this sort of zombie-ism as being similar to a form of insanity. The soul is still there, but the person is not fully in control of their actions, and thus not fully responsible for any "sins" they might commit while in the zombie state.
So the short answer to this question is "it depends." Knowing the genesis of a zombie is necessary in order to decide if it has a soul.
2) If the zombie is, in fact, a mindless, soulless, brain muncher, then I don't think there is any theological problem in destroying them. Their souls have already departed and met their fate, while the killers are not guilty of killing a human being per se, and thus, not sinning in the traditional sense.
If, however, the zombie in question is one of the ones mentioned above who may still have a soul, the waters get significantly murkier. The killer of this type of zombie would be facing a different theological situation. Since the zombie has a soul, this would technically violate the "no killing" rule found in most versions of the "Faiths of the Book". The exception would be in cases of self-defense or defense of the helpless, and of course, most zombie slayings are exactly that, so it's not likely to pose a problem for the faithful unless they begin to really enjoy the killing. Those who seek out opportunities to kill zombies would, I believe, be sinning in some way - if the killing is sought out for the sake of excitement or pleasure, you can hardly say that it is self-defense. I can envision a scenario where one might decide that the only sensible defense is to mount an offensive against a horde of zombies in order to "get them before they get us", but as we have seen on the international stage in recent days, this can be very sketchy, ethically speaking. Zombies generally have no weapons of mass destruction or major oil reserves...
The short answer here is "yes, as long as it's truly in self defense."
3) For the most part, the Abrahamic faiths hold that a person is judged for the sins they commit willfully - the insane, small children, and those who are forced to commit sins by way of torture or some other method of coercion are not held responsible for their actions. Without getting into Christian debates regarding the "once saved, always saved" doctrine, I think this means that if a zombie with a soul is killed, then he would have to be judged according to the state of his soul prior to zombification. If instead he is subsequently cured of the zombie curse he would, in effect, have a blank spot in his soul's history - the time during which he was a zombie would not be taken into account in his final reckoning.
The short answer for this question is "their souls go where they would have gone anyway." One is held responsible for all sins committed unless those sins are committed when one is not in control of one's actions.
I hope this sets your mind at ease when the killing commences. Your neighbors or loved ones who become zombies are not doomed to hell just because they ate some brains, and you are not doomed to hell for blowing them away in self-defense.
I welcome continued discussion on this topic - comment away.