A Pennington County jury has ruled that Walter Mousseau, 30, was not legally insane when he drowned Sheldon Glenn, 43, in Memorial Park on July 4, 2022. While Mousseau was found guilty of second-degree murder last week, the jury has now determined he was guilty but mentally ill—a verdict that holds him fully accountable while mandating psychiatric treatment during incarceration.
Under South Dakota law, this differs from a finding of legal insanity, which would have removed criminal punishment and focused solely on treatment. The defense failed to prove Mousseau was incapable of knowing right from wrong, and prosecutors cited his own statements and methamphetamine use—drug-induced impairment is not a valid insanity defense in the state.
Deputy State’s Attorney Emily Toms said the verdict reflects Mousseau’s awareness of his actions. He will be sentenced on December 22, 2025, facing a mandatory life sentence in the South Dakota State Penitentiary.
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