The law of self-defense justifies an act done in the reasonable belief of immediate danger. If an injury was done by a defendant in justifiable self-defense, he can neither be punished criminally nor held responsible for damages in a civil action. . . . One who seeks to justify an assault on the ground that he acted in self-defense must show that he used no more force than the exigency reasonably demanded. The belief of a defendant in an action for assault that the plaintiff intended to do him bodily harm cannot support a plea of self-defense unless it was such a belief as a person of average prudence would entertain under similar circumstances. The jury should accordingly be instructed that to justify assault and battery in self-defense the circumstances must be such as would have induced a rea()onable man of average prudence to make such an assault in order to protect himself. The question whether the belief of the defendant that he was about to be injured was a reasonable one under all the circumstances is a question for the consideration of the jury.