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Does murdering someone who is driving away from you prevent immediate harm to you? Uncontrolled moving vehicles with bricks on the accelerator are very dangerous.

Self–defence is extremely limited in scope. Basically, if someone is about to stab you, you can do anything necessary to prevent that stabbing. Self–defence doesn't include that after someone fails to stab you and runs away, you can shoot them while they are running away. It doesn't include revenge. If you shoot someone who fails to stab you and runs away, it is murder like any other shooting. Self–defence doesn't include shooting someone who invades your house and isn't currently trying to hurt you, that is castle doctrine.