A former U.S. marine accused of gunning down a man outside Guam Premier Outlets is pleading not guilty, by reason of insanity.

Gregory Arthur Romero faces a first-degree aggravated murder charge in the death of Keo Resse Cruz Taianao, shot multiple times outside the former Forever 21 at GPO.

According to police and court documents, Romero allegedly fired ten rounds at Taianao, three striking his chest. 

When officers arrived, they found Taianao face-up in a pool of blood. Romero was performing CPR, then raised his hands and admitted to the shooting.

Security footage showed Romero circling the area before confronting the victim. 

He's also charged with aggravated assault and faces life behind bars if convicted.

Romero is held on a $1 million dollar cash bail with his next hearing to be set at a later date.