So Self-Defense is in a class of defenses to a crime or tort called Affirmative Defense. Affirmative Defense is not the same of the defense that it was not possible for the Defendent to violate the law because of something factual that cannot (I.E. If the Defendant was out of country at the time the murder happened, than there is know physical way the Defendant could have committed the crime because he cannot be in two places at once.).
Rather, an affirmative Defense shifts the burden slightly in that the Defense must prove that the affirmative defense they claim would nullify an element of an incident that would make the events no longer meet all the required elements of the crime.
For simplicity sake, in a hypothetical jurisdiction the crime of murder in the first degree is defined as follows:
A person shall be guilty of the crime of Murder in the First Degree if:
1.) The person commits a homicide AND
2.) That the person showed planning to a degree to indicate prior planning
3.) That the homicide was unjustified.
That means, in order to convict, all three conditions must be met. If even one is not met, than the Defendant is innocent of this specific crime (They could be guilty of another crime, such as Murder 2 or Manslaughter or Attempted Murder, but they aren't guilty of Murder 1.).
An affirmative defense is an argument that the defendant did not commit the crime because, while some elements of the crimes definitely did occur, not elements occurred to make it this crime OR another law allows for an exception to this law.
As a general rule, in the U.S., Self-Defense permits for the use of force to defend ones self (or others including property) if they reasonably certain they will become victims of a crime with the minimum amount necessary to prevent the crime up to and including lethal force.
So with that in mind, Rittenhouse is arguing that he was about to be a victim of a crime, he had not provoked the altercation, and he used only the force necessary to prevent the crime.
Essentially, the trial was there to establish not if Kyle killed the person he was accused of killing nor if it was a premeditated killing as both those conditions are met, but rather if Kyle was justified in killing the deceased because he was preventing an imminent crime against himself.
In an ordinary trial with out an affirmative defense, Kyle is under no obligation to offer any evidence to support his case. It is the prosecutor who must prove that Kyle is did what he says Kyle did. If the prosecutor cannot do it, Kyle is "not guilty" and he doesn't have to say a thing (Rarely does this happen and it's rarer still that this strategy actually works for the defense when used). But if you use affirmative defense, you must admit some element of the crime did occur, and are now arguing that all elements of the crime occurred and must provide evidentiary support to that fact. That said, the burden of proof for a self-defense is still lower than the burden of proof for criminal conviction, so the prosecutor still must refute the evidence provided by the defense in order to convict. You may doubt Kyle's statements, but you may not convict him while doubting the prosecutor's own statements.
