This all refers to Use of Deadly Force. In many states justification or excusably will factor into the mix.
Fear in this circumstance = the reasonably held belief that another has the capacity, the will, the intent, that such action is imminent and would result in serious bodily injury or death to yourself and/or loved ones.
Dan Hawes (user on this forum) has said it more precisely, but the exact quote avoids me.
The reason why you can defend yourself is through the natural right to protect your property, your personal being being one of these things.
Fear as used in previous posts indicated the emotional feeling; Grape has provided a less or non-emotional definition. And even Grape's definition injects a inkling of a state of mind examination.
So either or both views or other factors too may = fear
And its why the law needs to be changed. It must be based not on state of mind assessments but of a clearly defined and measurable set of facts = fear.
And clarifying the law to include that any unwanted person on your land is subject to force is a reasonable rule and the facts related to this determination are generally easy to be assessed...and that's how laws should be, easily understood by all and easy to examine. I wonder how many people get convicted of crimes simply due to complexity of the evaluation of things that simply are not measurable.