In some states you are.
Many states have a “perception” clause that says even if there’s no overt threat to your life, if you perceive one from someone, you’re justified in using deadly force.
That applies only when you perceive the danger to your life to be at that present moment - not when you fear that someone may come back a day, a week, a month later to hurt you.
Again, if that was the standard, then you could kill anyone you wanted to - even if they were completely unarmed - and claim that you honestly believed that they were going to bring a deadly weapon to kill you the next day.