If you have been taught that in a self defense situation you should shoot to kill you have been misled. Firearms in the hands of sheepdogs and good people are meant to save lives, not take them away. Your objective when using them for self defense is to stop the threat. As I put it recently in a course of advanced students… first survive; then help others survive.
Perhaps you have been led to believe that if you don't shoot the bad guy they will survive to file a lawsuit against you. Two problems with this idea. First, so what? Is your money worth more than their life? Second, if you kill them their family will sue you anyway. Trust me, if you shoot someone you are going to face a lawsuit regardless if they die or not. Ask OJ Simpson about it.
Law enforcement officers are trained to first stop threats and then to administer first aid to all including those who they may have just fired upon. That is because they understand that they do not shoot to kill or shoot to take life, but always seek to save it. This is why even the best gun laws support the preservation of life with the use of deadly force. Taking life can only ethically be justified in order to preserve life.
If you ever wonder if you could really pull the trigger remember that you aren't shooting to take a life. You shoot in order to save lives. That is all the clarity and justification you will ever need.
Forget what anyone else may have told you. Your objective is not to kill, but to live. Your plan is not to take life but to save it.