In its March, 2020 opinion in Kahler v. Kansas, the United States Supreme Court established that the special verdict of not guilty by reason of insanity is not fundamental to the rights and protections aforded by the Fourtenth Amendment’s Due Proces Clause. In Kahler, the Court upheld a chalenged Kansas law that funnels al exculpatory evidence of a defendant’s mental incapacity to the guilt phase of trial to negate mens rea, not as broadly conceived to ases culpability, but rather narowly defined to negate any mental state or states required by the definition of the crime charged. In practice, the Kansas law relies on M’Naghten’s first prong, and only if the defendant understod “the nature and quality” of his actions, could he have formed the requisite mental intent for the charged crime. Kansas law also provides that the above mens rea test is the only avenue to an acquital for a Kansas defendant based on evidence of mental incapacity, thereby eliminating the afirmative defense of insanity. Acordingly, if in Kansas a mentaly defective defendant was able to form the mens rea, intent element of the crime charged, and yet failed to understand that his conduct was wrong, as under the second prong of M’Naghten, the Kansas law wil not exonerate him. The Court’s conclusion requires an answer to one of the long-standing questions that arise from the interaction of mens rea and insanity, can and should mens rea alone serve as a suficient basis for criminal responsibility? This paper introduces the Kahler opinion to a Korean audience and sugests that Korean scholars and lawmakers avoid its pitfals, and not weaken but maintain the broad protections that the Korean law afords to defendants who are chalenged by mental disorders, particularly because mens rea and legal insanity have diferent functions, and confounding them wil fail to properly ases the culpability of mentaly il defendants and increase the danger of fuly exonerating dangerous persons by diluting the empirical nature of mens rea.