The Master said:
First, do no evil. Then, do good. This is the Way.
The Student said:
What if I must do evil in order to do good?
The Master immediately struck the Student with his staff, knocking him senseless.
And it was good.
– The Book of Five Leaves
To me this feels like an example of the argumentum ad lapidem fallacy, the Master’s response is pithy and memorable but doesn’t actually answer the question. What if one must do evil in order to do good? Take a trolley problem scenario (a cliche I know but a useful one which quickly communicates the type of ethical dilemma I’m discussing), what if in order to do a good act one must also commit an evil act, albeit one arguably lesser in magnitude than the good act? Different ethical systems will have different answers with even more diverse justifications for their reasoning depending on the ethical axioms they follow and the values they hold. Bonking people on the head is not a useful way to discuss a legitimately complex and nuanced topic.
It’s a koan, not a formal argument in ethics. You have to contemplate it and let the deeper meanings unfold.
At the simplest level, note that the Master’s response was an answer to the question. He did evil [admittedly, a smack upside the head is traditional in many koans, but consider it from an Imperial perspective] in order to do good [further the progress of the Student towards enlightenment… at least once he wakes up and considers the deeper meanings of the throbbing lump on his skull and his instinctive anger].
(As a side note, as you may have gathered from discussions of trolley problems in the past, no-one there would consider it a terribly complex or nuanced topic so much as a good shibboleth for identifying people planning to warm up the atrocity factory. In the same sense as the old proverb that when someone starts talking about the greater good, it’s time to start looking around for the mass graves.)