Little is known about the underlying mechanisms of dual harm (DH), the co-occurrence of self-harm (SH) and other-directed harm (OH). This study examined the role of personality dysfunction, emotions, and situational factors. Using a vignette-based repeated-measures design, 319 participants with recent histories of SH and/or OH responded to scenarios manipulating five emotions (sadness, anxiety, guilt, dissatisfaction with self, anger) across three settings (alone, direct, indirect social contact). Participants rated the likelihood and severity of engaging in SH and OH, and personality dysfunction was assessed dimensionally using the PID-5-BF. Mixed-effects analyses revealed that higher personality dysfunction predicted greater harm overall, with a stronger association for SH than OH. Across conditions, SH generally exceeded OH, except in anger-related scenarios, which elicited higher OH. Internalising emotions were primarily linked to SH, whereas anger was uniquely associated with OH. Interpersonal setting influenced the strength but not the direction of harm, with significant emotion-by-setting interactions. These findings support the cognitive-emotional model of dual harm while suggesting that specific emotions differentially bias harm direction and that personality dysfunction amplifies vulnerability to harmful behaviour.