Lawyers can't lie ethically or even put a witness who knows he's going to lie. If a lawyer says something that he believes to be true but then learns that it is not, he is required to take steps to correct anything that resulted from the involuntary misrepresentation. The reason most criminal defense attorneys won't ask you if you're really guilty is because it's not relevant to the case. Besides, it's not your job to find out.
Their job is to defend you and present a fair case. As one lawyer said, your job is to maintain the honesty of the system. The way our legal system is structured, courts (judges and jurors) find the people responsible. Judges, not lawyers, are the ones who carry the gavel.