How do the political institutional features of developing democracies influence how violence occurs? Building on research showing that ‘hybrid democracies’ are more prone to social violence, this article argues that elite competition for power in the context of limited institutional oversight plays an important role in explaining violence. The framework here presents possible mechanisms linking subnational political dynamics and rates of social violence in poorly institutionalised contexts. It highlights how political competition, concentrated political power, and constraints on cooperation can create opportunity structures where violence is incentivised and the rule of law is undermined. This is examined empirically using sub-national homicide data from over 5000 Brazilian municipalities between 1997 and 2010. Findings suggest violence is greater in contexts that are highly competitive – where political actors face credible challenges and have a more tenuous grip on power – and those where power is highly concentrated – where political actors have held power for longer periods or face limited credible challenges. Findings also suggest violence varies depending on whether interactions between state and municipal government are likely to be constrained or cooperative; and are consistent with literatures emphasising the importance of structural explanations of social violence. In light of on-going democratic transitions across the globe, the article highlights the value of understanding links between institutional context, contentious politics and social violence.