We were taught in the academy that three things must be necessary for the use of deadly force (and why pull a gun if you don't plan to use it).
- Opportunity (the person is near enough to cause great bodily harm to yourself or another or commit a forcible felony)
- Ability (the have a means to commit that bodily harm or felony; ie, a gun instead of a knife when you're on the other side of a parking lot)
- An Overt Act (an act that would cause the great bodily harm such as drawing the weapon, raising it towards you, etc)