Our society, going back hundreds of years, has fundamentally considered one person intentionally hurting or killing another to be repugnant. Any such act of intentional violence against another human is prima facie a crime.
However, our legal system has recognized, also going back hundreds of years, that under some circumstances such an intentional act of extreme violence may be justified, thus relieving the actor from criminal (and civil) liability. But if you intentionally hurt or kill another person, your action is prima facie criminal unless/until it is established that the legal standard for justification was satisfied.
In some States an intentional act of extreme violence against another human might be justifiable in a broader set of circumstances. But the laws of no State can be considered a license to kill. If you intentionally hurt or kill another person, even in what you claim is self defense, you have prima facie committed a crime unless/until it can be established that your act of violence was justified.
"It is long been a principle of ours that one is no more armed because he has possession of a firearm than he is a musician because he owns a piano. There is no point in having a gun if you are not capable of using it skillfully." -- Jeff Cooper