How does witnessing regime atrocities influence the political attitudes of bystanders? We argue that observing regime violence against innocent civilians triggers psychological dissonance between beliefs about the regime and the witnessed moral transgression. As a result, regime support should decrease among bystanders of state atrocities. We analyze original, highly disaggregated archival data from the Nazi death marches at the end of World War II, which confronted ordinary German citizens with the regime’s crimes. We find that locations with higher victim numbers had lower vote shares for right-wing nationalist parties after the war. Supporting our proposed mechanism, we show that (1) this effect was strongest when Nazi crimes were at the center of public discourse and (2) that witnessing Nazi atrocities was associated with individuals’ rejection of Hitler 20 years later. The findings have implications for understanding democratization prospects and people’s nostalgia for fallen autocrats.