In what ways can words be dangerous like soft bullets?

Like a lot of women at that time, Mary has been falsely accused of witchcraft. The slanderous words used against her are incredibly dangerous; they can cause hurt and suffering, and they can lead to death by hanging, the standard punishment for a witch. In that sense, they are like soft bullets. These words are deadly and can kill, but unlike bullets the damage they cause cannot be felt straight away. That's what Atwood means when she refers to the metaphorical bullets of the false accusation as being "soft."
Yet they are no less dangerous for that. In such a tense, hysterical environment, when people see witches round every street corner, words really can kill just as surely as any bullet. The hanging itself does not kill Mary, but it destroys her psychologically. In that sense, the "soft bullets"—the lies, the slanders, the false accusations leveled against her—are indeed dangerous, and end up killing her soul.