John Henry Spooner was found guilty Wednesday of the first-degree intentional homicide of Darius Simmons in May 2012, the Milwaukee Journal Sentinel reported.
Spooner had confessed at the scene, saying he had "reached a breaking point" when Simmons had not confessed to stealing four of his guns.
During a sentencing hearing that began shortly after the verdict was announced, defense attorney Franklyn Gimbel argued Spooner was not guilty by reason of mental disease or defect.
Psychiatric expert Basil Jackson testified Spooner had a mental disorder he characterized as "hyper-manic personality" that caused a "sudden loss of control" which made Spooner unaccountable for the teen's death.
Assistant District Attorney Mark Williams said he would call two medical experts on Thursday who would testify that Spooner did not suffer from a mental disease.