Trial of James Holmes, who killed 12 people and wounded 70 at a crowded screening of the Batman film "The Dark Knight Rises" in Colorado in 2012, began on Monday.
The jurors will decide whether to give a life sentence to the gunman considering if he was sane or not at the time of the attack, when he shot at the moviegoers in the theatre with a handgun, shotgun and semiautomatic rifle.
Prosecutors are seeking death penalty for the former neuroscience graduate student, claiming he is a calculating mass murderer who deserves the execution.
In the opening statements, Arapahoe County District Attorney George Brauchler said: "One guy who felt as if he had lost his career, lost his love life, lost his purpose, came to execute a plan...He tried to murder a theatre full of people to make himself feel better," portraying Holmes as a cold blooded killer.
"When we're done getting all these horrific details ... I'm going to come back and stand in front of you, right here, and I'm going to ask you to reject that guy's claim he didn't know right from wrong," Brauchler said, showing images of the theatre with 400 people inside when Holmes attacked.
Holmes (27) pleaded not guilty to crimes claiming insanity.
Two forensic psychiatrists appointed by the court had studied Holmes and reported he was sane at the time of the shooting.
"This guy has a superior intellect ... he is smart," Brauchler said.
During prosecutors portrayal of the events in the theatre, relatives of some of the victims wept in silence in court.
Defence lawyers admit that Holmes was the gunman and killed 12 people in the theatre but claim he was experiencing a psychotic episode when he plotted and carried out the attack.
Public defender Daniel King claimed Holmes had schizophrenia and could not control his thoughts or actions at the time.
"By the time Mr Holmes stepped into that theatre, his perception of reality was so skewed, so malformed, that he no longer lived in the world we live in," King said.
"This is about a brain disease, a diseased mind, and about the power of psychotic delusion. The reality is mental illness.There's no evidence he's faking anything."
Holmes, who graduated with honours from the University of California, Riverside, had no criminal record prior to the shooting.
He had been seeing a school psychiatrist and withdrew from a graduate program at the University of Colorado's Anschutz Medical Campus before the attack.
Death penalties are very rare in Colorado and only one inmate has been executed in Colorado in nearly 50 years.