Now that I think of it.
Suppose it's the Others who re-animate the dead into wights.
How can they do that ? I suppose the Others are followers of The Great Other, the icy and evil god of darkness. I don't think we know his name, as his name should not be spoken. Just: The Great Other. The Great Other gives his followers the power to create zombies out of the dead. How terrible !
And the opponent of The Great Other is of course: R'hllor. The Lord of Light.
But wait a minute.

What does R'hllor do ? His followers are the Red Priests. Like Mellisandre and Thoros. Thoros is famous for his burning sword. But with us, the readers of the books, Thoros is famous for one more thing ! He raises the dead ! He raised Beric Dondarrion. And Lady Stoneheart. But to be honest, it doesn't make them "fully repaired". I don't see much difference between Lady Stoneheart and a zombie. Or a wight.
So basically, both camps have the same tricks ! Both R'hllor and The Great Other do the same thing. They raise the dead, to come fight for them.
So who is good, and who is evil ? And why again ?