The assessment of criminal responsibility in individuals with mental disorders represents a critical intersection between psychiatry and law. In Brazil, the biopsychological criterion is applied to determine criminal imputability, requiring both the presence of a mental disorder and its impact on the individual’s capacity for understanding and self-determination. This study analyzes the application of this criterion through a forensic psychiatric case involving a 42-year-old male diagnosed with delusional disorder who committed homicide motivated by religious delusions. A clinical-forensic evaluation was conducted, focusing on four key elements: identification of mental illness, establishment of a causal nexus, assessment of cognitive capacity to understand the illicit nature of the act, and evaluation of volitional capacity. Although the individual demonstrated formal awareness of the illegality of the act, his behavior was significantly influenced by a structured delusional system, impairing both judgment and self-determination. Based on these findings, the subject was considered criminally non-imputable and subjected to a security measure involving psychiatric hospitalization. The case highlights the importance of rigorous forensic psychiatric evaluation and reinforces the relevance of the biopsychological model in ensuring fair judicial decisions. Additionally, recent literature on delusional disorder and violent behavior supports the findings, emphasizing the role of delusional content in shaping criminal actions.