There is a primary and a secondary question to be answered here. The question of the propriety of self-defense must be answered, and to an ethical person the answer is not an automatic yes. To answer, it seems to me that the idea that a person retains life and wellbeing by privilege was placed beyond the pale some time ago along with the divine right of kings. This stance was well stated in 1776 and needs no defense here. I believe that self- defense is a responsibility more than a right on the grounds that it is immoral to allow unjust force to go unopposed.
Justice demands the opposition of injustice.
The harming of an inoffensive person would seem to be an injustice of the grossest kind, whether that person is yourself or someone else. If you allow an assault to proceed unopposed you become an accomplice, and share the guilt. We are all connected. It is bounded by that form of ethic known as appropriate force. Appropriate force is the gauge by which a response is measured.