Accordingly, Kahler appealed his case to the Kansas Supreme Court, raising ten issues related to the conduct of the trial and actions taken by the judge and prosecutor.

Two of these issues were the constitutionality of KSA 22–3220, the 1995 state law which abolished the traditional insanity plea and allowed defendants only the option of arguing that their mental illness prevented him from forming criminal intent, as well as the constitutionality of executing someone who currently suffers from a severe mental illness[10] In a ruling, the Kansas Supreme Court rejected Kahler's appeals, noting its prior precedent in State v. Bethel (2003): The same arguments made by Kahler were considered and rejected by this court in State v. Bethel, 275 Kan. 456, 66 P.3d 840 (2003).

Kahler relies on Finger v. State, 117 Nev. 548, 569, 27 P.3d 66 (2001), in which the Nevada Supreme Court held legal insanity is a fundamental principle of the criminal law of this country.

[5] His argument asserts that, for centuries, defendants were held culpable only when they were able to distinguish between right and wrong and that people who were legally insane did not have the capacity to do so.

The state also noted that the definition of insanity has varied in different ways throughout history and that one version (the M'Naghten rule) should not be viewed as an inherent aspect of due process.

[1] Kagan noted the need to refer back to eminent common-law authorities, such as William Blackstone, Edward Coke, and Matthew Hale.

[16] In the opinion, Kagan wrote that the Kansas law did not violate Kahler's fundamental right to due process, noting that definitions of legal culpability and mental illness have been traditionally reserved for the states.

Kagan noted that, contrary to Kahler's argument before the court, Kansas had not in fact abolished the insanity defense but had instead simply modified it, which the Constitution has generally permitted.

However, he argued that Kansas's law did not simply modify the insanity defense but had removed the core requirement of whether or not the defendant could distinguish from right and wrong.

[2] Breyer's dissent was rooted in the centuries of tradition behind the original M'naghten Rule and noted that only a handful of states had modified it in the way that Kansas had.