9. Act done by person justified or by mistake of fact believing himself justified by law

Nothing is an offence which is done by a person who is justified by law, or who, by reason of a mistake of fact and not by reason of a mistake of law, in good faith believes himself to be justified by law, in doing it.

Illustration

A sees B commits what appears to A to be a qatl. A, to the best of his judgment, and in good faith, in the exercise of the power which the law gives to all persons for apprehending murderers in the act, seizes B in order to bring B before the authorities. A has committed no offence though it may turn out that B was acting in self-defence.