RAPID CITY, S.D. – A Pennington County jury has determined that Walter Mousseau was not legally insane, but mentally ill, when he drowned 43-year-old Sheldon Glenn in Memorial Park on July 4, 2022, following up on last week’s second-degree murder conviction.
Under South Dakota law, this finding means Mousseau remains fully accountable for the crime and will receive a sentence like any other offender. The mentally ill designation, however, ensures he will undergo further evaluation and receive clinically indicated psychiatric treatment.
A finding of legal insanity would have removed criminal punishment, shifting the focus entirely to psychiatric treatment until the individual was no longer considered a danger to society, at which point the law would require their release.
In the insanity phase of the trial, the burden of proof rested with the defense, which was required to prove by clear and convincing evidence that Mousseau was incapable of knowing right from wrong at the time of the killing. The state argued that while Mousseau suffered from mental illness, the evidence did not meet the legal threshold for insanity.
Prosecutors highlighted Mousseau’s own statements to investigators and bystanders, which they said showed his awareness and intent. Toxicology results also indicated the presence of methamphetamine in Mousseau’s system at the time of the killing. Under South Dakota law, drug-induced impairment cannot be used as a basis for an insanity defense.
“The jury heard Mr. Mousseau’s own words and saw the full scope of his actions. Their verdict recognizes that he knew the wrongfulness of what he did. We appreciate the jury’s diligence and their commitment to a just outcome,” Deputy State’s Attorney Emily Toms said following the verdict.
Mousseau is scheduled to be sentenced on December 22, 2025. He faces a mandatory minimum sentence of life in the South Dakota State Penitentiary.
The prosecution team included Assistant Chief Deputy State’s Attorney Jason Thomas, Senior Deputy State’s Attorney Braedon Houdek, Deputy State’s Attorney Emily Toms, and Chief Deputy State’s Attorney Gina Nelson.
