Section 45 - Act done by person 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.

(b) A sees Z commit what appears to A to be culpable homicide. A in the exercise to the best of his judgment exerted in good faith of the power which the law gives to all persons of arresting murderers seizes Z in order to bring him before the proper authorities. A has committed no offence, though it may turn out that Z was acting in self defence.