5.2 Criminal Defenses Under Missouri Law

Defenses Are Not “Gotchas”—They Are Elements of Professional Investigation

Quick Answer: Under Missouri law, justification defenses claim the defendant’s force was legally allowed; excuse and related doctrines claim the defendant should not be blamed in the usual way. Peace officers document facts that support or undermine those claims—especially force, threats, retreat/options, officer vs private-person roles, and the suspect’s statements—without playing defense attorney or psychiatrist on scene.

MPOLE expects more than offense-element memorization. Many use-of-force and assault cases turn on who started the confrontation, what force was used, and whether the response was justified. Chapter 563 RSMo concepts (defense of persons and property) and related doctrines shape both civilian claims and how officers explain their own force later.

Think in two large buckets:

Officers do not “grant” defenses. Prosecutors and courts apply them. Your product is a complete factual record.

Justification: Self-Defense, Defense of Others, Defense of Premises

Self-defense (high-level Missouri concepts)

Self-defense generally addresses when a person may use force to protect against what the person reasonably believes is the unlawful use of force by another. Key themes for officers:

1. Reasonable belief — Not only what the actor subjectively felt, but whether the belief was reasonable under the circumstances as they appeared.
2. Unlawful force by the other person — Self-defense is not a license to assault someone who is lawfully present and non-threatening.
3. Proportionality / deadly force limits — Deadly force is tightly limited (commonly tied to reasonable belief of death, serious physical injury, or certain forcible felonies—know your academy’s Chapter 563 framing). Using deadly force over a minor shove or pure property dispute is a recurring exam and civil trap.
4. Initial aggressor problems — A person who starts an unlawful fight often faces limits on claiming self-defense, with nuances if they withdraw and communicate withdrawal.
5. Duty/stand-your-ground style issues — Training materials address when retreat is or is not required. Follow current Missouri statutory framing taught in your academy; do not invent a nationwide one-size rule on the exam.

Defense of others

Defense of others generally tracks the idea that a person may use force to protect a third person under circumstances in which that third person would be justified in using force in self-defense. Field implication: if you protect someone who was actually the aggressor, your “rescue” may not be justified. Investigate the full fight history, not only the last punch you saw.

Defense of premises / property (awareness)

Missouri law recognizes limited force to protect premises and property, but deadly force to protect property alone is rarely the professional or legal answer officers should expect to “win” easily. Typical awareness points:

• Nondeadly force may be more readily justified to terminate trespass or protect property under defined conditions.
• Deadly force related to dwellings/forcible felonies is a specialized statutory area—document exact location (dwelling vs open field), entry method, threats, and whether persons (not only property) were at risk.
• “He was stealing my bike, so I shot him” is a classic unjustified-deadly-force fact pattern in training questions.

Use of Force: Private Persons vs Peace Officers (Preview Distinction)

This distinction is tested because recruits confuse citizen self-defense with law-enforcement authority.

Private persons

Private persons generally may use force only under justification rules (self-defense, defense of others, limited property/premises rules, and any citizen-arrest authority that exists under Missouri law with its own limits). They are not armed with general “police power.” A private person who uses force is judged largely as a civilian actor under criminal justification statutes and civil liability rules.

Peace officers

Peace officers use force under a different operational and legal framework:

• Authority to seize persons under the Fourth Amendment and Missouri arrest law.
• Agency policy, POST training, and objective-reasonableness standards for official force.
• Tools and tactics (firearms, ECW, handcuffs) used as government actors.
• Reporting, review boards, and civil rights exposure under 42 U.S.C. § 1983.

Exam takeaway: An officer’s lawful arrest force is not analyzed identically to a bar fight between two civilians. Conversely, an off-duty officer in a purely personal dispute may be closer to private-person analysis depending on facts and whether they were acting under color of law. Document role, announcement, authority claimed, and purpose of force.

Excuse Awareness: Insanity and Competency (Not Deep Psychiatry)

Officers need awareness-level literacy, not forensic psychology credentials.

Competency to stand trial (process concept)

Competency asks whether the defendant can understand proceedings and assist counsel now. It is about present ability to participate in the case, not primarily about mental state at the time of the crime. Odd behavior at booking may trigger mental-health evaluation paths and medical care duties; it does not mean you “decide” competency on the curb.

Insanity / mental disease or defect (guilt-phase concept)

Insanity (mental disease or defect excluding responsibility, in Missouri framing taught in academy) concerns whether, at the time of the conduct, mental disease or defect meant the person did not know or appreciate the nature, quality, or wrongfulness of the conduct—exact statutory wording is academy-specific. Awareness points:

• Insanity is a legal defense, not a street diagnosis.
• Document bizarre behavior, statements about voices, medication status, and prior CIT contacts factually.
• Still secure the scene, render aid, and collect evidence; mental illness claims do not erase scene processing.
• Do not write “suspect is insane” as a conclusion; write observed behaviors and quotes.

Entrapment, Duress, and Necessity (Awareness)

Entrapment

Entrapment generally claims government actors induced the defendant to commit a crime the defendant was not predisposed to commit. Field relevance:

• Undercover and reverse-sting operations must avoid improper inducement narratives.
• Document the suspect’s ready willingness, prior related conduct if known lawfully, and who suggested the criminal idea.
• Private persons who tempt others usually do not create classic entrapment; entrapment focuses on government inducement.

Duress

Duress claims the defendant committed the offense because of a threat of imminent death or serious physical injury that a reasonable person could not resist (training-level paraphrase). Document:

• Who threatened whom, with what, and how soon.
• Whether the defendant had reasonable chance to escape or contact police.
• Consistency of the story with injuries, messages, and witness accounts.

Duress is narrowly applied and often fails when the threat is vague, future-only, or the crime is excessively serious under governing law—leave the legal edge cases to counsel.

Necessity (choice of evils)

Necessity (choice-of-evils style justification in many codes) claims the defendant committed a lesser harm to avoid a greater imminent harm. Classic training contrast: breaking a car window to rescue a heatstroke child vs. stealing a TV because “I needed money for rent.” Document the emergency, options available, and proportionality.

Mistake of Fact vs Mistake of Law

Mistake of fact

A honest and reasonable mistake of fact can sometimes negate the mental state required for an offense. Example themes: taking an identical bag believing it is yours may undercut purpose to steal if the belief is credible and reasonable. Officers should still investigate thoroughly—false “I thought it was mine” claims are common.

Mistake of law

Mistake of law—“I didn’t know that was illegal”—is generally not a defense. Society expects people not to commit crimes even if they never read the statute book. Rare exceptions (official misstatement of law, etc.) are not street-level free passes.

What Officers Document When Defense Claims Arise at the Scene

When someone says “I had to defend myself,” “they made me do it,” or “I thought it was my property,” shift into defense-aware evidence collection:

1. Separate parties and take independent statements before they align stories.
2. Photograph injuries, damage, distances, lighting, and weapons early.
3. Collect 911 calls, BWC, store video, and phone videos—sequence often destroys false self-defense claims.
4. Record exact quotes about fear, threats, and who struck first.
5. Note demeanor and inconsistencies without editorializing.
6. Identify initial aggressor evidence: prior threats, who approached whom, chase paths, thrown objects.
7. Medical treatment documentation for all injured persons, including the complainant.
8. Avoid debating the law with the suspect (“That’s not self-defense under 563…”). Investigate and report.

Mutual combat and “both want to press charges”

Many Missouri bar and parking-lot fights produce cross-complaints. Your job is not to pick a favorite relative. Build a timeline, assess injuries and weapons, apply probable-cause analysis to each actor, and consult supervisors/prosecutors when both sides have exposure. Dual arrests sometimes occur when both unlawfully used force; other times one side is clearly primary aggressor.

Linking Defenses Back to Charging and Court

Defense claims affect:

• Immediate charging decisions (assault vs. justified force / no charge).
• Bond and safety (ongoing threat vs. one-time defensive act).
• Trial themes (self-defense instructions are common in assault trials).
• Civil liability if officers ignore exculpatory facts or coerce statements.

Brady/Giglio-minded professionalism means you document exculpatory and impeaching facts, not only guilt facts. If witnesses say the complainant swung first, that goes in the report even if it complicates the case.

Exam Focus Summary

For MPOLE-style questions:

• Classify the claim as justification vs excuse/related doctrine.
• Apply proportionality and reasonable belief themes to self-defense items.
• Distinguish private-person force from peace-officer force.
• Treat mistake of law as generally invalid; evaluate mistake of fact against mental state.
• Choose answers that emphasize documentation and investigation, not officer-as-judge pronouncements on insanity or entrapment at the scene.