Say that you are an honest, moral person who happens to be a defense lawyer.
You want justice. Are you going to abandon a client because you THINK the MIGHT be guilty? No. But you're going to insist on the truth. You're going to make the prosecution prove it's case.
But say you come across a defendant who says "look, tell the DA I'll tell 'em where the child's body was dumped if they'll drop the death penalty. We'll talk about maybe 20 years or whatever. Offer 'em that."
So you do, and then the body is recovered. What do you do then?
No, sorry, you don't proceed along trying to get your client off. You don't try to do so by LYING. You KNOW he did it; he's told you so. So you're supposed so serve some sort of "greater good" by pretending that you really think some one else did it?
Sorry, this is just an untenable side of this argument. You people are trying to make this a noble thing, and it's not. It's just SLEAZY.