Except you don't enter a plea for an action, you enter a plea for a criminal charge. A murderer can confess a crime to his lawyer (under privilege), and still

Except you don't enter a plea for an action, you enter a plea for a criminal charge. A murderer can confess a crime to his lawyer (under privilege), and still honestly plead not-guilty to first-degree murder (for example, it could be considered manslaughter).