Sometimes no amount of discipline will address the issues these children have. Take care of yourself. Take care of your other children too. Before you know it, you’re spending so much time attending to the needs and blow ups of the one you’re neglecting the rest of the family (something that comes back to bite you in the butt years later at a family gathering). Please stay safe. It’s not normal for kids that young to have an infatuation with killing people.
Violent behavior in a child at any age always needs to be taken seriously. It should not be quickly dismissed as "just a phase they’re going through!" I have heard your story countless times, and I have unfortunately seen the results of parents taking this type of threat lightly. Sometimes there are other issues beyond your control. Don’t be afraid to reach out for professional help and supports.