I'd suggest that there's no moral failure if the person attacked responds with what is his/her best guess as to what is required to survive. I'd also suggest that nobody should be faulted for erring on the side of caution. In other words, the response to a sudden, violent, full-power attack which might result in my own death justifies an equally violent, maximum effort response which risks serious injury or death to the attacker. The latter, after all, has chosen his own course with his own assessment of the risks involved. Since I doubt that many of us will ever have the skill to kill with one blow, if you survive, you have controlled uke with some injury, most likely to both of you. If this restores some control to the situation, now is the time for moral choices--ie, a second blow, a coup-de-gras? How you make that choice requires courage and control. Uke may himself play a part in that decision, of course!