Answer: the correct answer is B. when we commit a crime, we teach others by our actions, and often the teacher is killed in the end.
Step-by-step explanation:
We still have judgment here; that we but teach
Bloody instructions, which, being taught, return
To plague the inventor...
Macbeth believes that if he resorts to murdering Duncan, there will be some earthly penalties waiting for him. Namely, if one commits some atrocity, one will stimulate others to do something alike. Killing Duncan may very well backfire on Macbeth once he becomes the king. Murdering someone powerful like Duncan may inspire others to harbor similar ambitions for themselves which could motivate them to do the same to Macbeth once he becomes the powerful leader. This is why he says that "bloody instructions, which, being taught, return to plague the inventor."