most violent crimes the argument can be made, "what normal right thinking person would do such a thing? He must be crazy"
But insanity has different levels and is viewed differently depending on the prism being used. There is a difference between medically insane and legally insane.
the term “insanity” itself has no precise definition, carries different meaning in different contexts and describes varying degrees of mental disorders.[
19] Every person who is mentally ill is not ipso facto exempted from criminal responsibility. A distinction is to be made between legal insanity and medical insanity. A court is concerned with legal insanity, and not with medical insanity.[
16,
19] Any person, who is suffering from any kind of mental illness is called “medical insanity,” however “legal insanity” means, person suffering from mental illness should also have a loss of reasoning power.
Insanity Defense: Past, Present, and Future.
So while the accused may well have a mental health issue, diagnosis, or history, it doesn't mean he was legally insane when he premeditated and carried out the execution of his three sons.