Will Trent co-showrunner Liz Heldens gives a detailed explanation of the season 2 finale twist. Emerging as a solid hit for ABC, and based on the novels by Karin Slaughter, the crime drama gets a boost from the relationship between the titular Trent and homicide detective for the Atlanta PD Angie (Erika Christensen). But their bond is complicated in the closing episode of season 2, which reveals that Crystal (Chapel Oaks) has been the one to go on a killing spree and that Angie assisted her at one point.
https://screenrant.com/will-trent-season-2-ending-angie-crystal-killer-explained-showrunners/