The real sting here isn't the warning against cruelty—it's that Nietzsche understood how *proximity* to evil can subtly rewire your moral compass without your noticing. A prosecuting attorney, say, who spends years studying criminal psychology can find herself adopting the very ruthlessness she's fighting against, not through malice but through the slow logic of her methods. What makes this different from a simple "don't be like the bad guys" is that Nietzsche knew the danger came not from deliberate choice but from osmosis, from letting your tools become your nature. The quote asks us to examine not just our actions but our *character* as it forms in opposition to something—because fighting monsters, he suggests, carries its own subtle infection.