What you gain unfairly eventually costs more than what you won.

An unfair gain arrives with something attached. Depending on the case it might be the ongoing effort of concealment, the knowledge of how you got it, a relationship damaged in a way that shows up years later, or the precedent it sets for what you will do next. The accounting is delayed but not cancelled.

Before taking an advantage that is not quite yours, ask what the attached cost would be and over what period you would be paying it. That question does not usually require a moral argument. Once the full price is visible, most of these opportunities stop looking like opportunities.