A person can be evil. Period. As in they do not regard any life to be sacred. You can call it a mental defect if you want, but to me, a person should only be able to use a mental defect defense to the extent it can be proven they did not know what they were doing. Which is not very common at all. Most murders know exactly what they are doing when they kill someone. And those, you are not going to change their minds thru therapy.