Why do some groups fighting in civil wars target civilians more than others? We propose an explanation that challenges the current focus on material and organizational factors and instead brings back and emphasizes the role of ideology. We argue that the ideological frameworks of armed groups, whether state or non-state, condition their decisions about targeting, in some cases setting normative constraints on action even if such choice involves higher costs and risks. We examine these hypotheses using a mixed-method approach that combines a statistical analysis of a newly constructed disaggregated data set on all fatalities in Northern Ireland's conflict between 1969 and 2005 with a comparative historical study of the interaction between key ideologies and the armed groups that adopted them.