YOU ALWAYS SHOOT TO KILL!
Some state's attorney's would take that statement as predisposition to kill someone. I thought you shot to STOP an attack.
Others say the main reason you shoot to kill is that dead people can't come back to try to kill you. Even if you were totally justified, and he goes to jail, every day he spends in jail, he is going to be plotting his revenge, especially if he now has to wear a colostomy bag or his face looks like Quasimodo. If he's dead, its highly unlikely a family member or a fellow gang member is going to mess with you.
Another reason to kill is that dead people can't testify against you. Its very unlikely that you will be charged if you say, he "threatened to kill me, he had a gun or knife or car, whatever, I felt in fear for my life" and there's nobody around to contradict your story.
Families of dead people sue every day, and dead people have friends who might want to harm you, so let's bury those two pieces of wisdom right now. Keep in mind that there is a lot more to an investigation that testimony.