The traditional conception of lone-actor terrorists is that they radicalize and commit violence alone. Recent work has challenged this view, identifying both extremist and non-extremist social relations as core aspects of lone-actor radicalization. The implications of this reinterpretation, however, remain underexplored. This paper combines a recognition of lone- actors as social beings and observed variations in social relations between far-right lone- and group-actors to sharpen our understanding of the radicalization and terrorism process across the whole population of far-right ideologically motivated violent actors. Specifically, it argues that variations in social relations can, to a degree, explain why some radicalize and/or act alone versus within groups, and develops a theory of the mechanisms through which this takes effect. This is achieved through the application of comparative process tracing to four case studies of far-right perpetrators, each selected from a novel typology of radical actors that further specifies the lone- versus group-actor distinction. This paper has important implications, both in terms of the approach used and the lessons it raises for counter-terrorism policy.