There’s no doubt John Joseph Delling knew what he was doing. His carefully planned 2007 crime spree lasted weeks, covered 6,500 miles and culminated in two people dead and one seriously wounded.Read the rest here.
He had his reasons, too. Delling, then 21, had become “a type of Jesus,” he later explained, and the men he attacked, two of them former classmates he had not seen in years, were stealing his “energy.” An MRI of his brain would have revealed the damage the men had already caused, he told authorities.
If it is possible for a human being to be so mentally ill that he/she really doesn't grasp what they are doing or the morality of it then yes, I do think that they or their lawyers have the right to raise that in court. Insanity is a very rare defense and it is almost always an "affirmative defense." By which the law means that the burden of proof shifts to the defense. I don't know of any jurisdiction where the prosecution is required to prove the defendant was sane. Normally as a condition of pleading insanity the defense has to first in effect concede that their guy did it and then they try to prove that he was several fries short of a happy meal and didn't know the difference between right and wrong. Since this is a very hard thing to prove usually an insanity defense only comes into play when when the evidence of guilt is so overwhelming that the defense basically goes for a legal "hail marry" pass. Or in those extremely rare cases where the defendant really is completely off his nut.
The insanity defense is a popular bugaboo. But the reality is that it is rarely attempted and then its success rate is extremely low.