I believe in the interconnectedness of all beings. In the great scheme of things your enemy is also your brother, and my life is no more or less important than his.
If he truly desires to kill you, then it is apparent that at least one life will be lost.
If you are in a position to defend yourself and you have searched your heart and found your intentions to be such that you are only defending yourself and must kill him in order to survive, then there is only one life lost and it is his. You are morally okay since he made the choice that one person would die.
The attacker has chosen to take a life, not you. Your actions are simply a tool that is used in response to the situation that he has created.
Because of the interconnectedness of life, I would feel sad at the loss of life. Sad for him and the unhappiness that brought him to attack me. Sad for his family that will miss him, and sad for his spirit.
I train to refine my skills in Aikido so that hopefully one day if faced with such as situation that I have the ability to choose to preserve life and not have to kill.
That is the only way we can be if we truly care to be good humans.
No one life is more important than another. It is wrong to kill, ever, unfortunately, "life" does not always care what we think or feel.