Peacemaker killed Rick Flag because Rick Flag intended to expose a government cover-up involving Project Starro. Rick Flag, who was the leader of Task Force X, discovered that the US government was secretly experimenting on an alien called Starro and planned to reveal this information to the world. Amanda Waller, who was in charge of the mission, directed Peacemaker to stop any leak of this information by any means necessary. Peacemaker tried to dissuade Flag but was ultimately forced to kill him in a brutal fight to prevent Flag from exposing the truth, as Peacemaker believed that revealing it would cause widespread disturbance and violence. Despite killing Flag, Peacemaker felt remorse and guilt over his actions.
Additionally, this event has repercussions leading to Rick Flag Sr. seeking revenge on Peacemaker in the Peacemaker TV series.
