This study investigates 30 lone actors in Sweden with a register-based design using a group of male lone actors and two reference groups: same-sex siblings and other male violent extremists. We compare lone actors to the reference groups along social background, criminal background, and co-offending relations (1995 –2016), and mental health (1980–2016). Our results show that lone actors are primarily born in Sweden to two Swedish-born parents. They have a high degree of criminality and co-offending, indicating that they are not completely loners in their criminal behaviour. They have higher enrolment in secondary education than the reference groups, but lower enrolment in higher education than other male violent extremists. Additionally, they suffer considerably more from mental disorders compared to the reference groups. An analysis of criminality and in- and outpatient hospitalisation over the life course indicates that lone actors may have had problems in their transition into middle age.