The current paper examines terrorism as a special form of moralistic violence, with several key features that distinguish such behaviour from other types of violence. The theory of lethal moralism highlights the importance of social polarization, characterized by vast differences in social space and inequality between adversaries as crucial to explaining deadly terrorist attacks. Where the differences are more permanent or chronic - and the groups in question define and justify their existence specifically in contradistinction to 'other' groups - then the polarization intensifies and attacks tend to be more lethal. In contrast, groups that appeal to broader audiences or the general public as potential allies more often use non-lethal terrorism to their strategic advantage. The study examines the United States and the United Kingdom to classify each of more than 8,000 attacks between 1970 and 2017 in terms of their ideological orientations. The evidence highlights the arc of terrorism in relation to different types of groups, as well as confirms the more lethal nature of terrorism linked especially to radical Islam, right-wing religious extremists, hate groups, ethno-nationalist sectarian violence, and anti-government anarchists. Yet apart from the extensive use of terrorism associated with 'The Troubles' in Northern Ireland, the majority of terrorist attacks in the US and the UK have not produced deaths. Most terrorism instead has been perpetrated by groups aiming to rally support for a general cause and has been far less deadly on balance. The implications of these findings are discussed with a view toward developing more powerful explanatory models that focus on the socio-cultural contexts and justification frameworks that inspire extremism and the use of lethal moralism to settle disputes.