Are there "bad" Witches who use their knowledge and power for personal gain and ill will? Yes, of course, just as there are "bad" Christians, "bad" Muslims, and so on. People are people. 

Just like everyone else, Witches confront issues that require them to make ethical choices. For instance, should Magick be used as a weapon, even if it's only to fight back?

Wiccans and Witches alike see Magick as ethically neutral, even as electricity is neutral. Both Magick and electricity can be used to help or harm. Magick is simply the intentional use of energy. The Witch directs energy by willpower toward a goal. How a person uses Magick - the Witch's intention - is what colors it White, Black, or Gray. To complicate matters, people's perceptions of what constitutes White, Black, and Gray aren't the same. Followers of any religion or belief system face a similar conundrum. Defining anything in concrete terms is nearly impossible.