The defendant in the Chris Kyle murder case argued he was insane at the time of the killing. There are various forms of the defense, but in essence it is the defendant’s inability to know that his actions are right or wrong, or that a mental disease somehow impaired his free will to act.
Normally, the burden is upon the defendant through his counsel to show that he was insane at the time of the murder. But in some states such as Colorado, the burden falls upon the prosecution to show that the defendant was sane at the time of the murder.