From Missouri Revised Statues, Ch. 571 - Weapons Offenses:
(this isn't the whole thing, only the applicable excerpts - Muzz)
Unlawful use of weapons--exceptions--penalties.
571.030. 1. A person commits the crime of unlawful use of weapons if he or she knowingly:
(4) Exhibits, in the presence of one or more persons, any weapon readily capable of lethal use in an angry or threatening manner; or
5. Subdivisions (3), (4), (5), (6), (7), (8), (9), and (10) of subsection 1 of this section shall not apply to persons who are engaged in a lawful act of defense pursuant to section 563.031, RSMo.
Here is 563.031 in a nutshell, as it applies:
From Missouri Revised Statues, Ch. 563 - Defense of Justification:
Use of force in defense of persons.
563.031. 1. A person may, subject to the provisions of subsection 2 of this section, use physical force upon another person when and to the extent he reasonably believes such force to be necessary to defend himself or a third person from what he reasonably believes to be the use or imminent use of unlawful force by such other person, unless:
(1) The actor was the initial aggressor; except that in such case his use of force is nevertheless justifiable provided
(a) He has withdrawn from the encounter and effectively communicated such withdrawal to such other person but the latter persists in continuing the incident by the use or threatened use of unlawful force; or
(b) He is a law enforcement officer and as such is an aggressor pursuant to section 563.046; or
(c) The aggressor is justified under some other provision of this chapter or other provision of law;
(2) Under the circumstances as the actor reasonably believes them to be, the person whom he seeks to protect would not be justified in using such protective force.
2. A person may not use deadly force upon another person under the circumstances specified in subsection 1 of this section unless he reasonably believes that such deadly force is necessary to protect himself or another against death, serious physical injury, rape, sodomy or kidnapping or serious physical injury through robbery, burglary or arson.