If someone wants to pay you to do something immoral, then that means that you're the best/cheapest/some mix of that they found. If you refuse, you make an immor

If someone wants to pay you to do something immoral, then that means that you're the best/cheapest/some mix of that they found. If you refuse, you make an immoral act more expensive or reduce its "benefits". That can be enough to reduce the cost/benefit ratio far enough that your potential customer does not consider it worth it anymore. If it does not, the next best option also has the option to refuse, and so on. Even if the immoral act eventually gets done, you have harmed the immoral actor, thus reducing their ability to act in the future -- and it's likely that you wouldn't approve of their future actions either.