What is the good / bad thing about the witches in the play?

The witches prophesy things to come, casting spells and doling out punishments. I've never considered them in terms of "good." Some consider their "goading" of Macbeth to be the reason he commits murder, some say it is fate, and I guess that it is possible their prophecy may have resulted in swaying Macbeth's choices rather than stating what will be his fate. We might consider the fact that instead of buckling to their instigations, he might just have chosen a different route.