If you are someone who likes to place bets, one safe bet is that the next mass shooting perpetrator will have a history of domestic violence. The majority of mass shootings tend to be domestic violence-related. In other words, the perpetrator is attacking a spouse, an ex, and/or other family members. In other mass shooting incidents, the perpetrator may not be targeting partners or family, but have a history of domestic violence in their past. That often shows in police records, etc. So keep that in mind when looking for risk factors.
The other factor that appears to interact with a history of domestic violence is consumption of violent rhetoric targeting specific social groups based on ethnicity, religion, gender/gender orientation, or sexual orientation. The recent mass shooting at an LGBTQ club in Colorado Springs appears to be tied to hateful rhetoric targeting the LGBTQ community, as well as legislation discriminating against our LGBTQ peers. Words on social media and elsewhere have consequences, and risk triggering those who already have histories of violence, and I would suspect histories of holding authoritarian attitudes. In the case of the latter, authoritarian aggression is the real concern. Authoritarians who believe that their legitimate authority figures consider targeting a minority group for violence is acceptable are at risk to follow through, and at bare minimum are at risk to approve such behavior.