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There are extreme generalities in what you are arguing that just simply aren't true:
I don't know the facts about who Holder defended beyond what I have read here, but I do believe there is an ethical difference between choosing to become a lawyer the bulk of whose time is spent defending corporations, and choosing to become, say, a public defender (who make up a large portion of those "defending" the prisoners in Guantanamo).
Many of the lawyers who represent Guantanamo detainees are lawyers who work for corporate law firms, who have taken those cases on a pro bono basis. Many of the "War on Terror" litigations have involved enormous resources to prosecute and I guarantee you that every lawyer at the ACLU, the CCR and other places will tell you that the willingness of partners at large corporate law firms to take these cases has been crucial in enabling them to be pursued.
Public defenders, in my experience, defend out of a deeply felt belief that, although most of their clients may in fact be guilty of their accused crime, their treatment by the state and the likely degree of punishment is deeply unjust. Corporate lawyers I've known present no such moral justifications for their jobs, and usually do it just as part of a self-interested career.
This is way too generalized to be meaningful. Some public defenders are as idealistic as you claim, but others hate their jobs but took stay there because it is all they could get, or because they see it as a stepping stone for doing something else, or because they don't want to work long hours. There's plenty of self-interest among every strain of lawyer, including public defenders.
Moreover, while Holder has been a partner in a large law firm, he has spent much of his career as a government lawyer. He's about to take an enormous pay cut -- going from DC law firm partner to AG -- and undoubtedly made far less than he could have in the private sector all those years when he was a DC Circuit Judge, a U.S. Attorney, and Justice Department official. Lawyers like him who work for the government do so at great financial sacrifice. That doesn't mean it's magnanimous -- lots of people trade money for power -- but it's nowhere near as clean and simplistic as you suggest.
Finally, it doesn't matter what the motive is. Lots of criminal defense lawyers represent accused criminals for money or other self-interested reasons. But it's still a vital service they're performing, and it's just wrong to suggest that the sins of a client can be attributed to the lawyer.