The concept of a magic spell "backfiring" on the one who had it cast is similar to the idea of "karma" as it is used in most modern American contexts. It conceives of magical spells (or certain types of magic spells) as being evil, and that a sort of spiritual punishment will befall the one who performed such a ritual or who arranged for it to be performed.
"Karma" in the sense of a mystic retaliation for negative deeds, is not a traditional part of hoodoo, nor of popular American spiritual belief before the 1970s or so. Traditional Eastern belief in the concept of karma holds it to be the force that governs what fortunes and misfortunes you will undergo in your life, these being the consequences of actions committed in your previous lifetime. (Basically, your good or bad deeds from a prior life create karma, which governs how you will be reincarnated in a future life.) One would not expect to suffer karma in this lifetime for a deed that was also committed in this life.
The earliest sources I have seen for a belief in this vein as far as the tradition of hoodoo and American folk magic are concerned, come from books published/written by the famous L.W. De Laurence. De Laurence was very influential and promoted himself with a reputation for being a great magician, although many of his books were mere reprints of public domain materials which sometimes had a few of his own additions and modifications. In his book The Great Book of Magical Art, he asserts the following:
The student and disciple should always bear in mind that in trying to demonstrate [magical energies] for himself, he is working with unseen and powerful agents, which, if he is not equally powerful -- pure and high-minded, loving to his fellow men, and seeking to benefit mankind, rather than seeking or desiring powers and benefits for himself, he had much better be dead than to try any of these things for the gratification of his personal nature; for in seeking to harm another -- curses like chickens soon come home to roost, with a much greater force than the original impulse.
Interestingly, this does seem to be a passage that De Laurence wrote himself. He goes on to promote one finding his "higher self" when turning to magic. It has a lot in common with typical Masonic beliefs. Elsewhere in his books he promotes concepts like karma and other Eastern religious notions, but these are usually more similar to their original concepts than the dumbed-down versions that are popularly known by Americans today.
In general American, Christian-influenced belief, there was an understanding that bad deeds (magical or not) were potentially punishable in the afterlife by God or other divine forces, but one might not receive comeuppance for evil deeds in this life except by the intervention of other, virtuous men. Even then, depending on the situation, magicians understood that doing harm might not be the same as doing bad. Afterall, the Bible is full of stories of God’s wrath, and if He’s on your side and allowing your spells to succeed, you’re doing good as good gets. The place where it starts to get muddy is in the belief held by some Christians that all magic is inherently evil or Satanic. Seen through this lens, it is natural to believe that any magic will end in punishment; and also, if governed by the Devil, it is normal to believe that he will tend to double-cross or will deliberately manifest the spells in an undesirable way, just to be malicious.
Hoodoo tradition, however, even flips this on its head. It's interesting to note from Harry M. Hyatt's collected hoodoo folklore that even those practitioners who believed in interacting with the Devil to perform magic, also believed in double-crossing him. One popularly repeated spell among Hyatt's informants (modeled off of a ritual in the Cyprian grimoire) was that to become a magician, one had to offer his or her soul to the Devil; but, in a particularly hoodoo twist, it was reported that if the Devil should ever come to collect the debt, all one had to do was rip the sole off of a shoe and present that as the payment, which would sneakily fulfill one's promise of a soul/sole without really giving up anything of value.
Nowadays, many modern practitioners of magic are Wiccan or else are heavily influenced by Wicca. This religion was developed in the 1930s and 1940s and possibly drew on De Laurence's popular magical books, as well. Wicca is a very decentralized religion and so its beliefs will vary greatly from person to person -- but for that reason one cannot say that Wiccans or "witches" all believe any particular point of it. Many a dabbler also mistakes things seen in Hollywood movies and TV for being an official part of the religion -- for example, a belief that witches can't perform spells for personal gain, seems to derive more from the TV series Charmed than from actual mystic texts.
In the end, hoodoo is a type of folk magic with no particular religious belief tied to it, and so it’s up to you and your own religious or personal reservations to decide what is wicked work or not. But remember -- if you are hiring a spell caster to work magic on your behalf, the caster's own beliefs are all that matter as far as what they're comfortable performing and what sort of consequences (if any) they'd expect you to receive.
WHAT TO DO WHEN YOU HAVE USED THE WRONG SPELL
If you still think karma or backfiring is an issue, here is my advice on what to do. There are generally two courses of possibility:
- Stop and/or undo the existing spells that were cast.
- Continue working spells but add another spell to counteract the negative effects.
Whatever the case, DO NOT PANIC. Stay calm -- if possible, pretend like you don't even know there's a problem, until such time as you have your solution devised.