What is the defence of self-defence in a murder case?

In Regina v. Rose (1884) 15 Cox 540, the accused was charged with the murder of his father. The defence was based on the accused's honest belief that the killing was necessary to prevent the murder of his mother. In the course of his summing up, Mr.Justice Lopes said at p. 541: " Homicide is excusable if a person takes away the life of another in defending himself, if the fatal blow which takes away life is necessary for his preservation. The law says not only in self-defence such as I have described may homicide be excusable, but also it may be excusable if the fatal blow inflicted was necessary for the preservation of life. In the case of parent and child, if the parent has reason to believe that the life of a child is in imminent danger by reason of an assault by another person, and that the only possible, fair, and reasonable means of saving the child's life is by doing something which will cause the death of that person, the law excuses that act. It is the same of a child with regard to a parent; it is the same in the case of husband and wife. Therefore, I propose to lay the law before you in this form: If you think, having regard to the evidence, and drawing fair and proper inferences from it, that the prisoner at the bar acted without vindictive feeling towards his father when he fired the shot, if you think that at the time he fired that shot he honestly believed, and had reasonable grounds for the belief, that his mother's life was in imminent peril, and that the fatal shot which he fired was absolutely necessary for the preservation of her life, then he ought to be excused, and the law will excuse him, from the consequences of the homicide. If, however, on the other hand, you cannot come to that conclusion, if you think, and think without any reasonable doubt, that it is not a fair inference to be drawn from the evidence, but are clearly of opinion that he acted vindictively, and had not such a belief as I have described to you, or had not reasonable grounds for such a belief, then you must find him guilty of murder. "