Casey did not claim self defense.<quoted text>
Shame on you -- you're not a newbie. The BURDEN resides with THE STATE to prove to the jury that a defendant is guilty beyond reasonable doubt, by presenting evidence which consists of testimony and exhibits.
Have you forgetting Cheney Mason's closing arguments to the jury - the defense is NOT required to prove a defendant's innocence and is legally entitled to rest without presenting any evidence at all!
The burden of an affirmative defense falls on the defense.