A good witch is someone who helps people using knowledge gained from experience handed down through many generations (kinda like folk music). As a lot of this knowledge is not known by ordinary folks it was/is considered mystical or magical.

A single example will suffice, the wise ones, witches, healers, or whatever you like to call them were using mouldy bread to cure infections many hundreds of years before Alexander Fleming 'discovered' penicillin.

Because, long ago, this kind of knowledge was not known or accepted by those in power in Europe, namely The Christian Church. The Church had to get rid of these people who were a threat to THEIR knowledge which they were foisting on the ordinary people. SO, they said that these wise ones were bad and that their knowledge was the work of the devil, and they killed them because they were BAD WITCHES.