Whether you kill him right away depends on your skill level and your ability to control the situation.
The ethical answer, IMHO, is to use minimum force necessary to resolve the situation.
It is awful hard not to get caught up in your emotions in this kind of situation.
Ideally you would be so well trained that you defend yourself without regard for life or death, calmly and with compassion for your attacker. Once the situation is resolved you would attempt subdue your opponent, or help him, give him first aid or whatever until the authorities get there, you would show him whatever kindness you could.
That aside, you attempt to suppress anger, revenge, or fear. When you become un-aiki is when you pass the point of resolution, and you enter into revenge and "over do".
My goal in life is to refine my skills so that I can perfectly resolve the situation with no harm done whatsoever. Where I am today is probably somewhere in the "he would be hurt very badly, or dead" stage.