Given that morals are basically internalizations and formalizations of mores, the basic societal expectations or social contract, they cannot be truly individualized. They definitely share a broader context whether the foundations be cultural, religious/spiritual, or "other" in nature.
I think the true test here of whether something is "moral" or not (in this case the legally justified but morally questionable killing of an assailant) is whether you would freely and proudly admit to it in your town square, without reservation. If you have reservations about doing so, it probably isn't moral. If you feel you can proudly say, "No I didn't have to kill him to protect myself, but since I wasn't legally obligated to retreat out my back door, I was legally justified in doing so, I figure 'What the heck?'", then go right ahead......
. Be prepared for outrage.........
Since the killing of others is frowned upon when it can be avoided, then I would say we definitely have a moral as well as legal duty to retreat if retreat can be accomplished safely. The duty ends when "safely" ends.