It's the government's job to establish the guilt of the accused. It is not the defendant's advocate's job to help out. The actual guilt or innocence of their client is not a concern—legally speaking. Ensuring that they get a fair trial, and are afforded all the process due to them is.

A defense attorney will poke whatever holes they find in the state's evidence, in their theory, and in their conduct. They will work to suppress evidence that was gathered illegally, locate witnesses that can testify on behalf of their client, and argue before the jury that the state has not proven their case beyond the legal standard.

I'm sure it's much easier for lawyers when they themselves are convinced of their client's innocence. But when someone comes to you and pays you to advocate for them in court, that's what you do. Your personal opinion is irrelevant.