• 2012 Aurora mass shooting killer James Holmes

2012 Aurora mass shooting killer James Holmes (Photo : REUTERS/ANDY CROSS)

James Holmes, the man behind the 2012 Aurora mass shooting, has expressed regret over one of the deadliest mass shooting in America, which took place during a midnight showing of "The Dark Knight Rises" inside a Century movie theater in Aurora, Colo., on July 20, 2012.

Holmes "now regrets what took place in the theater," according to his defense attorney, The New York Daily News reported.

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Citing reason of insanity, Holmes has pleaded not guilty to killing 12 people and injuring 70 when he opened fire at a Batman movie screening in suburban Denver.

According to his attorneys, Holmes could not tell right from wrong when he went on the shooting spree while he was under a psychotic episode. The death penalty trial depends on what was going on in Holmes' mind at the time and not whether he was responsible.

On April 27, Monday, attorney Daniel King said there is "no rational explanation for what took place. He said in his opening statements at Holmes' murder trial that his client displayed strange behavior in a jail cell in November 2012.

Showing a video of Holmes running head first into a wall, King said he also found Holmes licking walls while he was naked. He told the jury, "Look at the video, and you tell me if he would do this for notoriety."

On the other hand, the shooting spree appeared to be premeditated, according to prosecutors, considering that Holmes amassed an arsenal including a shotgun, a rifle, body armor, two pistols and tear gas canisters, according to The Wall Street Journal.