Individuals frequently witness acts of ostracism (e.g., in schools or in workplaces). Research examining the impact of vicarious ostracism is still in its nascent stages and has not yet explored how observing ostracism might affect observers’ perceptions of human nature and antisocial inclinations. And only to a limited degree have the moderators of the effects of witnessing ostracism been identified. The present study sought to fill these gaps in the literature by determining whether observing ostracism might lead observers to subsequently perceive human nature in general less favorably (i.e., to see people as more immoral and less trustworthy) and heighten their antisocial inclinations. It further examined if these effects were moderated by observers’ dispositional attachment. I hypothesized that, after witnessing ostracism or inclusion relative to a neutral control condition, anxious attachment would predict reduced favorability of observers’ perceptions of human nature and increased antisocial inclinations. In contrast, avoidant attachment was expected to predict enhanced favorability of observers’ views of human nature and reduced antisocial inclinations, but only after witnessing ostracism. Six hundred and twenty-seven university students observed another individual be ostracized or included in an online game of ball-toss or anticipated an upcoming game. Subsequently, I assessed their views of human nature as well as their antisocial inclinations. Results partially supported hypotheses. After observing ostracism observers low in attachment anxiety were marginally more likely to report less favorable views of human nature (and a similar trend emerged after observing inclusion), while witnessing ostracism significantly increased the antisocial inclinations of observers high in attachment avoidance. Discussion focuses on the meaning and theoretical and practical implications of these results.