Just as I think Nazis should have the right to free speech, someone accused of the most heinous crime imaginable should also have the right to a legal defense and for that defense to advocate in the best interests of the defendant, even if that legal team is fully aware that they did the thing they are accused of doing.

IMO (and IANAL), I think that the ethics of the situation demand that lawyers represent the best interests of their clients in civil and criminal matters.