No, it doesn't have to be life in immediate danger with castle doctrine laws which Missouri has. THIS is where you and other posters keep getting it wrong. Castle Doctrine allows for use of lethal force to defend PROPERTY as well as life. Someone breaking into your house to steal your valuables, but not "harm" you can still be legally shot. Why? Because the laws for castle doctrine don't try to go into minutia bullshit of trying to define every little action in a situation for what may or may not be an imminent threat. It is obvious that someone is in a place the legally shouldn't be and knew they weren't suppose to be there. That is why castle doctrine allows for any actions by property right owners to defend their property up to and including lethal force. Brandishing or Exhibiting a firearm is a legal use of force within castle doctrine laws. Technically warning shots are also legal as well, and are easier to defend in court in cases of castle doctrine laws (much easier) than when it comes to self defense laws only. While legal though in self defense situations, it can give an to n over zealous prosecutor to inject that by firing a warning shot you didn't think it was completely life threatening in that self defense situation. Every so often, the prosecutors win that, but it isn't as often as one may think. The real problem with warning shots, is it may not discourage the person and instead encourage them to actually kill you before you can decide to fire something more than a warning shot. It just isn't worth it.
SCOTUS has already ruled the mere carrying of a firearm can't be legally deemed as a threat or brandishing. Period.