You can get some additional mileage from the fear and unease if the fictional situation is familiar, one in which the audience may find themselves at a future date. I am not ordinarily afraid of the dark, but I have found myself jumping at shadows after a late night session of Left 4 Dead. So much of the game takes place inside houses that it is difficult not to carry something of the tension away.
The Ring also made a strong impression on me, probably because every scene involving a certain device becomes a vehicle for horror. I expect I will never look at a television tuned to static in quite the same way again. The shift to digital television means fewer chances for this, of course, but that only enhances the terror.
I thought the ending for Anchorhead worked really well, somewhat better the one for The King of Shreds and Patches. In both, I think the intent is to undermine or negate the player’s victory. Anchorhead lets you save Michael, then suggests that he is not quite free from dark influences. Shreds and Patches lets you save Lucy, then foreshadows the eventual return of Hastur’s cult, long after you both are dead. Clearly the first is more terrible; the second may as well be happily ever after: it holds out the possibility of an exciting sequel, but does nothing to diminish the player’s sense of triumph.
It seems hard to pull off moral absolutism these days. An evil character, to me, is one that shares the same relative morals as the audience, but who chooses to act in an immoral fashion. By that standard, the main character in De Baron is perhaps the most monstrous in all of IF.
If your zombies are intended to say something about what it really means to be a monster, then you cannot really avoid a discussion of evil. But it may be more convenient to simply treat zombies as amoral, and let the terror come from interacting with creatures that shrug off human norms.
An interesting approach might be to make a player character with a strong moral compass, like a priest, who undertakes to save and purify the zombies. Perhaps he succeeds in cleansing them, but certain zombies subsequently relapse and resume feeding on human flesh. The question becomes whether these are simply immoral degenerates, or if the others have the same urges but more self-control. The player must decide whether he will kill the zombies or redouble his efforts to save them.
Whatever the outcome, the terror comes from the need to act, then to live with the uncertainty. Has the player perpetrated a great evil, or allowed a great evil to flourish?
The average player accepts the necessity of killing zombies without much trouble. It could be rather fascinating to get the player to question that fundamental assumption, and to ask when it becomes OK to kill someone for being different.
If you move the struggle to a level beyond “kill or be killed”, if the zombies pose no direct physical threat even as they work to summon a hostile cosmic power, if it is the player that must initiate violence against the unresisting zombie horde, what is the ethical choice?
Perhaps good and evil only apply to human conflicts; here zombies are uniquely well-suited to showing us what counts as human. Perhaps it is evil and yet must be done; here the terror comes from inhabiting a world that forces this duty upon us.