Witches have a long history in folklore. The term witch usually refers to a female practicing magic, but some wiccans use the word for all genders. Witches were often women with knowledge of herbal remedies and who also acted as midwives, healers and wise-women but the term commonly used has many culturally specific models and is difficult to pin-down any specific traits. Essentially though, it relates to women who is able perform 'Magic' or has secret knowledge which they use to harm another or gain control over others. Combined with a patriarchal and religious society, it is easy to see why they were deemed to be dangerous or harmful by many in that society.
There have been numerous witch-scares throughout European history, causing the deaths of countless women and the Bible verse of Exodus 22:18 ("Thou shalt not suffer a witch to live") has been used as justification for this mass-murder. Across Europe in the middle-ages and into the modern era sorcery, black magic, witchcraft, summoning the Devil and various other practices where crimes that would lead to death. It is estimated that at the height of the Witch-hunting phase in Europe (a few decades either side of 1600) 50-70,000 people were burned at the stake for the crime of witchcraft. The vast majority were women over the age of 40.
Sadly the accusation is still prevalent in societies with poor education and high religiosity. In Central Africa it is often the children that suffer from these accusations, especially if they are physically different. Any child with albinism (lack of pigmentation in the eyes, skin and/or hair) is especially susceptible and many - even very young children - have been abandoned by their families or killed by mobs; often encouraged by evangelical, pentecostal-type preachers who blame these innocent kids for bringing curses, illness and even natural disasters to their community. Ironically, 'witch doctors' also prize their body parts for use in magic rituals and many have been killed in this way too