Acquittals
In a united-states court, it is essentially never proven that a particular crime did not occur, or that a particular act was not a crime. An acquittal means that the jury did not accept that the charge was proved beyond a reasonable doubt. The jury members (or some of them) may have thought it significantly more likely than not that the accused was guilty, but not quite sufficiently proven.
Self Defense
When a defendant argues that an act was taken in self defense and was therefore not a crime, and presents some evidence tending to establish this, the prosecutor has (in many states including NH) the burden of showing beyond a reasonable doubt that the act was not one of self defense. If the jury is unsure whether it was self-defense or not, they should, under the law, acquit. Such an acquittal in no way "proves" that the act was one of self-defense.
Self defense is considered an affirmative defense in many states, but not in New Hampshire (see Munroe Below). But a prosecutor must generally prove beyond a reasonable doubt that the act was not self defense, once the issue has been plausibly raised (see sources below).
A Jury's Reasons
Moreover, a jury never gives reasons. A jury does not acquit "BECAUSE the jury decided his action was not criminal" or "because they found self defense" or because anything. The only question asked of the jury (in a criminal case) is "guilty or not-guilty" (if there are multiple counts or multiple included offenses, they are asked guilty or not guilty on each). A jury may acquit for any reason, or none. If the jury thinks that another person might have committed the act, it can acquit on that basis, even though the defense presented was one of self defense. The judge is not permitted to ask the jury why it reached the verdict that it did. (FRCP Rule 32 permits polling the Jury to verify a unanimous verdict, but no other questions. State rules are similar on this point.)
Precedent
Precedent, mentioned in some of the comments, can be binding on matters of law in similar circumstances, but never on matters of fact. Besides which, trial courts do not create binding precedents, even on matters of law. Only appellate courts create binding precedents.
Burden of Proof on Self Defense
New Hampshire
In THE STATE OF NEW HAMPSHIRE v. MICHAEL MUNROE No. 2018-0433 (2020) The NH Supreme court wrote:
The Criminal Code specifies two types of defenses: defenses and affirmative defenses. See RSA 626:7, I (2016). Defenses, also referred to as pure defenses, see, e.g., State v. Soucy, 139 N.H. 349, 352 (1995), are those defenses that the State has the burden of disproving beyond a reasonable doubt. ...
...
... Self-defense is a pure defense under New Hampshire law. See RSA 627:1 (2016) (“Conduct which is justifiable under this chapter constitutes a defense to any offense.”). Thus, “[w]hen evidence of self-defense is admitted, conduct negating the defense becomes an element of the charged offense.” Pennock, 168 N.H. at 307 (quotation omitted); see also Soucy, 139 N.H. at 352-53.
Maryland
The decision in Jacobs v. State 32 Md. App. 509 363 A.2d 257 (1976) says:
As Chief Judge Murphy made very clear for the Court of Appeals in State v. Evans, 278 Md. 197 (1976), the allocation of the burden of persuasion on self-defense in Maryland has been affected by Mullaney v. Wilbur:
The court concluded in Evans that a jury instruction given in a homicide case which cast upon the defendant the burden of persuasion by a preponderance of the evidence when the issue of self-defense was raised by the evidence (see Wilson v. State, 261 Md. 551, 276 A.2d 214 (1971)) was constitutionally defective under Mullaney since it shifted the burden to the defendants to prove justification or excuse for the homicide. We agree with that conclusion and therefore hold that all Maryland cases which contain contrary holdings are no longer valid precedents.
...
In our Evans v. State, we analyzed at length the constitutional inappropriateness of placing upon a defendant the ultimate burden of persuasion with respect to what had been classified traditionally as "affirmative defenses." We there said, at 707-708:
This was the rule throughout much of the common law world with respect to the so-called `affirmative defenses.' The defendant bore the ultimate risk of non-persuasion. If the minds of the jurors were in a state of equipoise on the existence or non-existence of self-defense, hot-blood, entrapment, etc., the defendant lost because he had not carried his burden of preponderating persuasion. Casting upon a defendant that sort of burden of ultimately persuading the jury of his innocence (by negating in one fashion or another the necessary criminal element of a guilty mind) was the thing condemned in Mullaney v. Wilbur and Winship. ...
Under these guiding principles, we hold that where self-defense has been fairly generated by the evidence as an issue in the case, the burden is upon the State of negating such self-defense beyond a reasonable doubt as a necessary element of its proof of guilt.
Washington State
In "Who Carries The Burden Of Proof For Self-Defense?" from the law officwe of S. W. Thayer, it is said in regard to State v. Acosta 101 Wn.2d 612 (1984) that:
The defendant was accused of second-degree assault. That crime requires the state to prove beyond a reasonable doubt that the accused knowingly inflicted grievous bodily harm. Therefore, the government is constitutionally required to prove beyond a reasonable doubt that the defendant knowingly committed the unlawful act. It is the prosecutor’s burden to prove that the defendant did not act in self-defense. The Washington Supreme Court agreed and reversed the conviction.
(Note that Thayer was one of the defense lawyers in this case.)
The actual opnion said:
The issue in this case is whether, in a prosecution for second degree assault under RCW 9 A. 36.020(1)(b), the State must disprove a defendant's claim of self-defense. The Court of Appeals held that the State need not do so, and affirmed petitioner's conviction. State v. Acosta, 34 Wn. App. 387, 661 P.2d 602 (1983). We reverse the Court of Appeals and remand for a new trial.
The due process clause of the fourteenth amendment to the United States Constitution requires the State to prove beyond a reasonable doubt all facts necessary to constitute the crime charged. Sandstrom v. Montana, 442 U.S. 510, 99 S. Ct. 2450 (1979); In re Winship, 397 U.S. 358, 90 S. Ct. 1068 (1970). There are two ways to determine whether absence of self-defense is an element or ingredient of the crime which the State must prove: (1) the statute may reflect a legislative intent to treat absence of self-defense as an element of the crime; or (2) proof of self-defense may negate an element of the crime. State v. McCullum, 98 Wn.2d 484, 490, 656 P.2d 1064 (1983); State v. Hanton, 94 Wn.2d 129, 132, 614 P.2d 1280, cert. denied, 449 U.S. 1035, 66 L. Ed. 2d 497, 101 S. Ct. 611 (1980).
... Even if the Legislature did intend to require the defendant to prove self-defense, this requirement will withstand constitutional scrutiny only if we determine that self-defense does not negate an essential element of the crime. McCullum, at 494; Hanton, at 132. ...
Self-defense is defined by statute as a lawful act. See RCW 9 A. 16.020(3). It is therefore impossible for one who acts in self-defense to be aware of facts or circumstances "described by a statute defining an offense". RCW 9 A. 08.010(1)(b)(i). This is just another way of stating that proof of self-defense negates the knowledge element of second degree assault. Since proof of self-defense negates knowledge, due process and our prior cases require us to hold that the State must disprove self-defense in order to prove that the defendant acted unlawfully.
...
If we were to hold that the defendant bore the burden of proving self-defense, we would be relieving the State of its obligation to prove that the defendant's use of force was unlawful. Therefore, consistent with our prior cases, we hold that in a second degree assault prosecution the State must disprove self-defense beyond a reasonable doubt. State v. LeBlanc, 34 Wn. App. 306, 660 P.2d 1142.
Federal
FEDERAL BURDEN-OF-PROOF RULE IN SELF-DEFENSE CLAIMS says:
In federal court the prosecution has the burden of disproving self-defense, once the claim has been properly raised. This rule does not appear in either Title 18 of the federal statutes or the Rules of Criminal Procedure but is established by common law. (In the federal system many of the defenses, such as duress, entrapment, and self-defense, are common law defenses.) The federal standard is that once a defendant meets the initial burden of producing sufficient evidence for the judge to give a jury instruction on self-defense, the burden shifts to the government to disprove the defense beyond a reasonable doubt. (See United States v. Thomas, 34 F.3d 44,47 (2d Cir. 1994).)
Other States
In some states, including New York but not NH, a defendant must prove self-defense by a preponderance of the evidence, but never beyond a reasonable doubt. So even in those states a successful use of self-defense evidence to obtain an acquittal does not "prove" that the act was one of self defense, and does not preclude conviction (in a separate trial) of others involved in the same acts.
