Background: Opioid maintenance treatment (OMT) is regarded as a crime control measure. Yet, some individuals are charged with violent criminal offenses while enrolled in OMT. This article aims to generate nuanced knowledge about violent crime among a group of imprisoned, OMT-enrolled individuals by exploring their understandings of the role of substances in violent crime prior to and during OMT, moral values related to violent crime, and post-crime processing of their moral transgressions. Methods: Twenty-eight semi-structured interviews were undertaken among 12 OMT-enrolled prisoners. The interviews were audio recorded and transcribed verbatim. An exploratory, thematic analysis was carried out with a reflexive and interactive approach. Findings: Prior to OMT, substances and, in particular, high-dose benzodiazepines were deliberately used to induce 'antisocial selves' capable of transgressing individual moral codes and performing non-violent and violent criminal acts, mainly to support costly heroin use. During OMT, impulsive and uncontrolled substance use just prior to the violent acts that the participants were imprisoned for was reported. Yet, to conduct a (violent) criminal act does not necessarily imply that one is without moral principles. The study participants maintain moral standards, engage in complex moral negotiations, and struggle to reconcile their moral transgressions. Benzodiazepines were also used to reduce memories of and alleviate the guilt associated with having committed violent crimes. Conclusions: Substances are used to transgress moral codes prior to committing and to neutralize the shame and guilt experienced after having committed violent crimes. Being simultaneously enrolled in OMT and imprisoned for a (violent) crime might evoke feelings of 'double' shame and guilt for both the criminal behavior prior to treatment and the actual case(s) one is imprisoned for while in OMT. Treatment providers should identify individuals with histories of violent behavior and, together with them, explore concrete episodes of violence and their emotional reactions. Particular attention should be given to potential relationships between substance use and violence and treatment approaches tailored accordingly. What appears as severe antisocial personality disorder may be partly explained by substance use.