Sam Stone series Series by Jack Hood

Klaus Hartmann, a dual citizen from Hamburg, Germany, who has a dark triad of psychological problems, moves to Alabama where he becomes romantically involved with Lauren Kemper, a banjo player. Both enjoy outdoor sports, especially shooting Glock pistols. Unfortunately for them, deception, misplaced expectations, and violence, lead to a mass killing. Sam Stone, a former federal prosecutor, plays a key role in the legal outcome.