Here’s a fairly cogent explanation from non-defender Randy E. Barnett of why defense lawyers should keep fighting even for people who aren’t factually innocent. The meat of it is this:
Criminal lawyers are constantly asked how they can live with themselves defending those guilty of serious crimes. The full and complete answer ought to be that, because we can never be sure who is guilty and who is innocent until the evidence is scrutinized, the only way to protect the innocent is by effectively defending everyone.
Barnett’s “full and complete answer” is missing a hugely important component: punishment. Anyone found guilty (legally guilty, as opposed to factually guilty) should expect to have a lawyer fighting to minimize his punishment. I would add this to Barnett’s formulation:
Because we can never be sure who deserves what punishment, the only way to protect those who don’t deserve the crushing weight of the government’s retribution is to effectively defend everyone.