Despite the amount of research on partner aggression, the field lacks a unified theory. Personality problems, substance use, marital distress, and psychological aggression are consistently included in models of physical aggression but these variables have not been incorporated into one concise model with a strong theoretical rationale. We hypothesize that psychopathic traits, borderline traits, and alcohol use problems are each risk factors for psychological aggression and physical aggression during relationship conflict. We also hypothesize that borderline traits suggest poorer relationship quality due to the interpersonal difficulties central to Borderline Personality Disorder, which in turn is a risk factor for greater psychological aggression. Finally, we suggest that psychological aggression is likely to escalate into physical aggression regardless of the associated psychopathology. This study tests a model of physical aggression in a sample of 194 men in treatment for physical or psychological aggression in the Madrid area of Spain. Results suggest the prevalence of aggression in this Spanish batterer sample is lower than in batterer samples in the US. In the path model highlighted in this paper, borderline traits were risk factors for poor relationship quality, psychological and physical aggression perpetration, and alcohol problems. Alcohol problems were associated with physical aggression. Psychological aggression and alcohol problems significantly mediated the association between borderline traits and physical aggression. These findings are discussed within a culture-sensitive context and we also address conceptual and treatment implications.