I think the term originally was a clear and reasonably precise description of a specialist, found in many different African societies, whose job, or part of it, was to protect against malign witchcraft. In different African societies these specialists were given different names in local languages, but the English term was clear, and covered them all. In Zulu such a specialist is called an isangoma, and that term has been universalised in the form of "sangoma" to apply to other societies too. Another way of translating "sangoma" into English (though it is well on its way to becoming an English word in its own right) is "diviner". The diviner is not only a witchdoctor, but rather determines the cause of evils and misfortunes, such as disease, quarrels, accidents, crop failures and the like. The cause, as determined by the diviner, may be witchcraft, but it may also be that the ancestral shades (amadlozi in Zulu) are annoyed because they have been neglected. Witchcraft is not the only possible explanation for misfortune.
If witchcraft is determined as the cause, then the sangoma may put on his (or her) witchdoctor hat, and prescribe treatment. This may include the use of umuthi (Anglicised as "muti"), in which case the sangoma is functioning as a herbalist or medicine man (inyanga in Zulu).
A witchdoctor, therefore, is one who protects against the harmful activities of witches. One of the articles in The Pagan Activist, however, implies that "witchdoctors" are the ones who perform harmful activities uxsually attributed to witches, which implies that witchdoctors actually cause harm, rather than preventing it.
I suggest that there are two possible sources for this misunderstanding.
- Hollywood movies, especially those of the mid-20th century, which portrayed "witchdoctors" as a force of evil, tyrants in African societies, and especially likely to turn a tribe against white visitors. Some of this may be based on historical incidents. When the Voortrekker leader Piet Retief visited the Zulu king Dingane in 1838 to negotiate a treaty, the king ordered his soldiers to "kill the witches" (bulala abathakathi). It is possible that a diviner told him that Retief and his companions were witches; it is also possible that he reached that conclusion on his own.
- A witchdoctor who "changes sides" and practices as a witch. A parallel can be found in Western medicine with a medical doctor who misuses his knowledge to poison and kill patients. Such cases are not unknown. Also, a corrupt police officer might moonlight as a member of a criminal gang. This does not, however, mean that "doctor" means "poisoner" or that "policeman" means thief"; so it also does not mean that "witchdoctor" means "witch". And sometimes sangomas may use their specialist knowledge in activities that are beneficvial to some, but harmful to others. In a recent case a four-year-old child was murdered on the advice of a sangoma, and parts of the child's body built into the wall of a hairdressing saloon, as muti to make the client's business prosper. Ritual murder, however, is not witchcraft.
I won't go into the different meaning applied to the word "witch" by many neopagans. That is another discussion, and one that I have dealt with in an article on Christian responses to witchcraft and sorcery.