Why do mass-killing episodes end? Most attention is paid to international tools for ending atrocities. Instead, we consider how domestic politics alter the duration of killing by focusing on how divisions within the regime may lead to coups during the violence. Coups help shift regime preferences and undermine capacity to continue killings. We find support for this argument by statistically analyzing the relationship between coups and the end of each mass-killing episode from 1946 to 2013. We explore each mechanism quantitatively, and buttress these results with a series of examples illustrating the mechanisms at work.