Sincerely, you believe this? Join the Taliban.
I'm speaking as a lawyer. To slightly paraphrase the law in New York, just as an example, someone is guilty of murder if he intends to kill a person and kills that person or another person. But he is not guilty if he reasonably believes that killing that person is necessary to defend himself against unlawful deadly force by that person.
So, yes, the difference between murder and self-defense is defined purely by the defendant's mental state.