I think it depends on whether we are talking about business or personal ethics. The idea that a law firm makes money for its services while presumably looking out for the best interest of their client doesn't really present an ethical dilemma. In most cases, providing the best services possible is the way to earn money. Basic business models tell us that in a competitive market, you have to provide the best services to remain competitive and continue to gain clients and make money. Depending on the case and situation, it is not always as simple as that, but I don't think there is a huge ethical dilemma. The one exception might be death penalty cases. The cost of dealth penalty legal representation is astronomical and not likely to provide a profit to the lawyer.
As far as personal ethics: I think lawyers face this dilemma everyday. Do you take money to strenuously represent a client who may be guilty? Once you become their lawyer it is your job to represent them as best you can. It is what you are being paid for and it is the law. If you as a lawyer have moral or ethical issues with your clients behavior, it can be a difficult road to travel.