Grand juries are used only if a prosecutor believes a crime has been committed. You may be thinking of a coroner’s jury.
How many times have we heard of self-defense cases all over the country being referred to grand juries by the prosecutor? Many... when the home defender is non-LEO, he has to overcome the objection that he had no choice, or was in fear of his life, or prove that he was (in the better states) just "standing his ground" as opposed to "lying in wait."