We live in a world of opposites: high-low, cold-hot, night-day. We tend to believe that what isn't good is bad. It would be good-bad. However, in this case, it's not like that. Let me explain with an example. You meet someone you find pleasant and become their friend (the good). As time passes, that person begins to change, according to your interpretation, and you're right. You based your opinion on appearances, but appearances are deceiving because they are just appearances (the not good). So, you decide to distance yourself because you believe it's for the best. That person gets angry with you; they can no longer make you their prisoner or possess something from you that they believe they lack and that you can provide. But they can no longer do so because you've left their life. They need to recover what you, according to them, took from them. In this feeling of being hurt, obviously, according to them, it's your fault, they get angry with you and wish you ill (the bad). She might do anything to punish you because, in her opinion, you deserve it. For example, she might cast a spell on you (the bad thing).
However, there's a huge difference between not being good and actually harming someone or wishing them harm.