Deadly force is force likely to cause death or serious bodily injury.
Deadly force is only allowed in response to deadly force.
With deadly force, a duty to retreat may apply.
The majority of states have no duty to retreat. One may stand their ground wherever they are legally allowed to be.
Retreat states say that one must retreat if he can do so safely.
The majority of states have a castle doctrine, which says that there is no duty to retreat within one's home.
Some states also apply the castle doctrine to person's business or car.