A couple questions: is it ethical to lobby for a company actively trying to do bad? Is it ethical to represent a serial killer?
Let me take the serial killer question first. “Represent” is ambiguous here, but I take it to mean legal representation, such as defending him at his murder trail. In that case, I would say yes, it is ethical. In fact, if you are a public defender, and called upon to represent him, I would say it is your ethical obligation.
The thing is, the integrity of the American court system rests on the idea that everyone gets roughly equal treatment, and roughly equal access to the same resources, chief among them being competent professional representation. If we allow even the worst people to be convicted and sentenced without access to those resources, then the integrity of the system is undermined.
Now, if the CEO of Charlie’s corrupt corporation is on trial, and the question is whether or not to represent him in court, my answer would be the same. Lobbying, however, is an entirely different matter. Nobody has a right, legal or otherwise, to have their interests professionally represented on Capitol Hill. And whereas legal defense is a necessary function of the justice system, corporate lobbying can often have a corrosive effect on the democratic process. Monied interests too often wield disproportionate influence over lawmakers and other government officials, leading them to make decisions that are in contradiction with the interests of their constituents and the advice of less compromised experts.
So far as lobbying for corporations that do significant harm to the public interest goes, I would say no, that is not ethical.