Satanism is often considered to be an identical or a related practice to Voodoo, which itself is often considered to be a synonym for evil magic. The popular thinking is that "Satanic witches" will perform "voodoo ceremonies" -- involving sacrifices of animals or humans -- as part of Satanic worship, and may use the rituals to cast spells meant to work wicked intentions like death or persecution of the Satanist's perceived enemies; or sometimes the spells are intended to cause very morbid ways of growing rich or gaining love, like causing wealthy relatives to die so one gains their inheritance or to turn someone into an enslaved "zombie lover" with no free will or significant mental capacity.
Voodoo and Satanism are actually two separate religions -- and not very compatible ones, at that. Most of the major Satanic churches are actually atheist, whereas traditional African Voodoo is a monotheistic religion but wherein many lesser spirits are bestowed with power to help (somewhat akin to the concept of the Christian angel.) Haitian style Voodoo is that, with even more Catholic influences -- and New Orleans style Voodoo even more Catholic still.
While some Voodoo ceremonies might involve animal sacrifices, these are not inherently indicative of "evil" being done. Don't forget that even the Bible makes mention of animal sacrifices to God in early stories (Abel's favored offering of lambs being a good example.) It is a well-known way to worship deities in many parts of the world.
Beyond just the stories of sacrifices, accusations of human cannibalism by Voodoo practitioners go back to at least the 19th century. It's sometimes speculated that early transports from Africa may have attempted such rites, but even the Journal of American Folklore acknowledged its researchers could find no actual recorded case of arrests or charges for voodoo-related cannibalism in the US. It is notable that European witches were often accused of similar deeds, but no evidence beyond the witches' coached and coerced "confessions" given under torture seem to ever suggest that it really occurred. Even early Christians were accused of cannibalism by the Romans due to a misunderstanding of the meaning of "the body of Christ."
As for "Evil spells" -- while Voodoo practitioners might cast spells, it is not the primary point of the religion to do so. Christians can cast spells as well, and Christian Witchcraft has a long history of existence. Both Voodoo and Christian magical practitioners might cast harmful magic spells, either against their religion, or through loopholes or certain interpretations that view harmful work as positive in some circumstances -- for example, as being in line with the "wrath of God" so as to punish bad deeds in others.
Ultimately, these things should be remembered:
- Voodoo is not Satanism
- Evil magic is not necessarily Voodoo
- Voodoo practitioners do not necessarily use magic