The thing is, evil denotes sentience. How can a plot of land be "Evil"? How can, for that matter, an animal be evil? Even if a specific animal is the most horrendous thing and creation and it slaughters everything that moves, is that really evil, or it is just an accident of nature that this thing is so dangerous and that it kills and eats everything else.
Moreover, if undead are mindless, can they be evil? Their creation might be a perversion of nature, but the undead itself isn't necessarily evil. If it has a mind and is intent on causing harm, it can be said to be evil, but there are plenty of evil humans out there, and they don't live on "evil" land surrounded by gloomy trees and evil animals, etc. It is the individual creature that is evil.
So to have an "Evil" area you'd have to have some kind of sentient force that exerts its will over the place and inflicts evil upon it somehow. I think it's easier to handle this simply with entities (demons, sorcerers, etc.) rather than an "evil" zone that just makes so-called "evil" stuff spontaneously appear.
Having zones for "death", "chaos blight", "faeries" and so on makes more sense I think. If a place is going to be evil it should be so because a nasty creature came out from the underworld and set up shop there, or a wicked sorcerer, or a demon, maybe enslaving people and teaching them to slaughter outsides and present their heads as sacrifice, possibly spawning nasty or dangerous creatures through magic or twisted genetic propagation of some kind. In this case it would simply be a product of the local entities making evil things happen.
Something like "death" or "chaos blight" would be more in line with properties of the land, I think. Faerie as well- if the particular area is some how connected with other realms of existence, or there is something weird in the soil that affects the ambient life there, then that works as a zone, not the result of an entity. Perhaps "good" and "evil" entities can tend to gravitate towards areas that foster that kind of behavior more, but I think it is a bit too clumsy to tie a single area to these concepts.