The insanity defense should be abolished. It is not possible to prove that someone did not know the difference between right and wrong when they committed a crime. There are many reasons that the insanity defense should be abolished. Both personal responsibility and freedom should be emphasized in society. The legal system should not fail to enforce just laws because of unprovable beliefs and statements, like that someone is insane and therefore not responsible for their actions.
Psychiatric slavery should be abolished, and psychiatric slavery includes both the insanity defense and civil commitment. Psychiatric slavery is like chattel slavery in some ways, but different in others. Chattel slavery was eventually abolished, and psychiatric slavery should be abolished, too. There was a time when there was wide spread support for chattel slavery, and that eventually changed. Eventually, the majority of the humans on Earth may be against psychiatric slavery as well.
Hopefully the insanity defense will be abolished sooner rather than later. There is a separation of religion and state in many countries. There should also be a separation of psychiatry and state. The late Dr. Thomas Szasz has written many books on why civil commitment, the insanity defense and psychiatric slavery are all bad, wrong and should be abolished. More individuals should read books by Dr. Thomas Szasz. There is no point in reinventing the wheel and he has laid forth many quality lines of reasoning as to why the insanity defense and psychiatric slavery should be abolished.
Another way to think about this is in the context of religious motives versus insane motives. When some humans plan to commit a crime because they believe God wants them to be violent, their sanity is not questioned and they charged under terrorism/criminal laws, and when other humans plan to commit crimes because they claim/believe God wants them to be violent, they are declared insane and dealt with not under the criminal system, but they are diverted to psychiatric systems. They should all be treated equally and they should all be treated under the criminal system. If they have not broken any laws then they should be left alone.
All adult humans should be held responsible for their actions. There is simply no way to prove that someone is truly insane and therefore not responsible for their actions. It would be like trying to prove that a certain religious is true or that unicorns or ghosts exist. It cannot really be truly done, despite that many humans believe in various religions and/or ghosts.
If someone is breaking or has broken a law, then the criminal justice system should deal with them. If one has not broken any laws, then one should be able to enjoy the right to be left alone and not bothered. The insanity defense is part of psychiatric slavery. The other aspect of psychiatric slavery is civil commitment. That should also be abolished. Psychiatric slavery should be abolished.