The mistake a lot of people make is to assume that the good/bad is inherent in the magic itself whereas of course magic is actually a neutral force which works to serve the intent of the witch.
The intent of a witch is dependent on the nature of the individual, the circumstances the individual may find his/herself in and the changing nature of his/her moods. Very few people are absolute in the sense that they always act with total bad or total good. Most of us make bad decisions sometimes, lash out in temper, hurt people or act in our own self interest at the expense of others. Witches are no different – practising witchcraft doesn’t change the intrinsic blueprint that makes a person what they are – the vast vast majority of witches as with anybody else have a blend of both good and evil in their hearts.
There are witches who choose to exclude their practise to either good or evil. Well I say that but what I really mean is that there are witches who choose to use their magic for positive purposes. You’ll hear a witch term herself a “white” witch by which she means she focuses exclusively on non harmful magic but you’re much less likely to hear a witch dedicate herself to “evil” as it is human nature for a person to believe they are acting for the best of reasons. So even if a witch is happy to use her craft for harmful and “darker” purposes that might be considered “evil” by others it is more than likely that she can justify what she is doing to herself as being the right thing to do.
So to sum up my answer – witches can be good, bad, either, neither or both.