You cannot both ask him to do his best, and complain that he does his job too well. The defence lawyer has a necessary, and entirely, moral part to play in the process. Moreover, that very morality requires someone to suspend judgement of a defendant. Someone is needed who neither believes nor disbelieves the version of events being put forward by the defendant, but puts them forward as strongly as he or she can. This mode of 'beliefless' assertion is very strange.
Many people see it, or choose to see it, as lying.
People think that we lie for clients. I am sure it happens, but it happens rather less than people think. Indeed, it is not at all unusual for a lawyer to say to a client 'You have just said that 'A' is true, but also that you want me to say that 'A' is not true. A client who asks his lawyer to lie for him should, and will usually, lose his lawyer. Many times things are not so simple.