Gallo, 23, was accused of driving drunk and speeding when he ran a red light and crashed into a car, killing three of its four occupants, in Fullerton on April 9, 2009. He also was accused of fleeing the scene before police found him two miles away running against traffic on the shoulder of a freeway.
The driver, 20-year-old Courtney Stewart, and 25-year-old Henry Pearson died instantly. Adenhart, 22, died later in surgery, just hours after pitching six scoreless innings in his Angels season debut. Another passenger, Jon Wilhite, survived, but had to have his skull reattached to his spine.
In her closing arguments Thursday, the prosecutor told jurors that Andrew Gallo "didn't care about anyone else" when he took the lives of the three victims. Calling it a "very simple case," the prosecutor said the evidence clearly shows Gallo is guilty of second-degree murder.
Gallo's defense attorney argued that her client was charged with a higher offense because of the victims' notoriety.
The prosecutor told jurors she never showed them pictures of the three victims because this was "not a case about sympathy, but it's about the suspect's actions."
The prosecutor told jurors Gallo's actions were intentional, he and his stepbrother each had several beers and shots at three bars before the crash. His blood alcohol level was nearly three times the legal limit.
The prosecutor argued Gallo knew the dangers of drinking and driving. He was on probation at the time because of a 2006 DUI conviction and was driving on a suspended license.
Gallo's attorney told jurors he's not a murderer and had no intention of driving that night. She said Gallo was in an alcohol-induced blackout and lost his sense of right or wrong and did not intend to kill anyone.
"Everybody thought his stepbrother was going to drive, including the stepbrother," said defense attorney Jacqueline Goodman.
The defense argued Gallo may not have been driving, insisting his stepbrother's facial injuries could be consistent with the damage on the driver's side windshield.
The prosecutor said Gallo himself admitted he was the driver and told jurors Gallo didn't care about anyone but himself, pointing out after the crash he told his step brother to run.
"This is horribly difficult for the families by participating and being in court. They're reliving the loss of their loved ones," said Michael Fell, a victim rights attorney. "Four families were horribly devastated. We trust in this trial, justice will prevail."
In order to convict Gallo, jurors must decide if the defendant acted with implied malice: intentionally drove drunk; acted with a conscious disregard for human life; and knew from his personal experience that he could kill someone.
Gallo has pleaded not guilty to three counts of second-degree murder, hit-and-run, and two other alcohol-related felonies. If convicted on all charges, he could spend up to 50 years behind bars.
The Associated Press contributed to this report.