http://www.nononsenseselfdefense.com/lethalforce.html
Immediate, means "at this very second."
There is literally nothing more dangerous to you and your family than *not* understanding what is meant by "immediate" or "imminent" (depending on which term your state uses). This idea cuts through *all* emotions, fears, thoughts and suspicions and defines when you are - in the eyes of the law - justified to use lethal force.
If he isn't trying to kill you right now, you aren't justified to use lethal force.
It doesn't matter if he is standing there screaming and threatening to kill you, or if has said that he is going to come back and get you or -- in many states -- has just pointed a gun at you, demanded your wallet and is now running away -- those are not considered "immediate threat of death or grave bodily injury." Because he isn't trying to kill you at that exact moment.
Not understanding the meaning of this term will put you in prison for murder. At the very least it will endanger everything you own to litigation....and, odds are, you will lose if you pulled the trigger at the wrong time.
In theory, someone standing across the room waving a knife threatening to kill you isn't offering you an immediate threat. Which means that you cannot legally shoot him. On the other hand, when he starts charging across the room, then you are in immediate and immanent danger of death or previous bodily harm. The reason being is that a knife is a close range weapon and by rushing at you, he is now capable of harming you. Now granted his brandishing the weapon in a threatening manner is in and of itself a crime, but not enough to warrant shooting him.
Now that is theory, in reality this is somewhat of a grey area. Not only does it depend on whose lawyer is better, but also the laws of the particular state (or country), what the legal precedents are there and what is the current local interpretation is.
In one court you might be acquitted for shooting him while he is drawing the gun, whereas in others you will be convicted if you shoot before he has fired the first shot.
At the very best of times it is a very, very slippery slope.
Unfortunately, a situation that has spun so far out of control that deadly force was used, is very seldom the best of times.