"Astonishingly, the events of Aug. 31 and Sept. 1 did not temper the defendant's dangerous, arguably aggressive and out-of-control behavior," Assistant District Attorney Mark J. Conte wrote in his memorandum to the judge.
The defense maintains that O'Neill, 18, has himself been victimized by Sheridan's death, becoming reclusive and despondent - an ideal candidate for treatment in the juvenile system.
"He just feels very, very isolated from everyone," forensic psychologist Bruce Mapes testified, adding that O'Neill has described a "repetitive dream where he's sinking in wet sand or mud."
At stake is the potential punishment: supervision under the juvenile system that would end when O'Neill turns 21 in less than three years vs. a maximum five-year penalty for manslaughter and a 20-year maximum for aggravated assault.
Chester County Court Judge Thomas G. Gavin is expected to decide the issue as early as this week. He will rule based, in part, on five days of testimony by a succession of mostly teenage witnesses who described unsettling activities that preceded and followed the death of Sheridan, 18, of West Chester.
Mike Puliti, who said he was friends with Sheridan and O'Neill, testified that O'Neill brandished a gun at him about 14 hours before shooting Sheridan.
Puliti said O'Neill and some friends arrived at Puliti's house about noon on Aug. 31 so Puliti could cut O'Neill's hair. Puliti said the unloaded gun belonged to another friend, Colin Higgins, and was displayed as a "joke."
Later, during the unchaperoned drinking party at O'Neill's home, several participants described handling a loaded .45-caliber semiautomatic pistol, which O'Neill pulled from under a mattress, and going for a ride to use it. (That gun was owned by O'Neill's father, Sean O. O'Neill Sr., who would later face charges for different weapons found in the house after the Sheridan shooting.)
"We all were all for it," testified Pete F. Gallen IV, of Newtown Square, who admitting shooting at cars owned by people the boys disliked between 1 and 2 a.m. on Sept. 1.
Gallen, 20, testified that Sheridan - whose fake ID had enabled the teens to buy beer - wanted Gallen to shoot a vehicle at White Horse Retirement Village in Newtown Square. Sheridan worked there and wanted to hear about it the next day, Gallen said.
He testified that things were getting "a little crazy," so he fired into the ground and lied that he shot a car.
After the shooting spree, O'Neill, Sheridan, Gallen, Andrew F. Miller, now 19, of Glen Mills, and Charles "Chuck" Mulloy, now 17 and a student at Penncrest High, returned to the O'Neill residence. Gallen testified that his mother called about 2 a.m., and he went home.
The party was down to the four of them, Miller said, and they were all out on the driveway. O'Neill had the pistol. After Miller heard a shot, he observed Sheridan on the ground, "a bit twisted up." A single, mortal bullet had hit Sheridan in the face.
Miller said that he remembered seeing "a blank stare on Sean's face" and that O'Neill, who hosed blood off the driveway a few minutes later, said "he would rather die than go to jail."
Asked by Conte whether anyone discussed calling 911, Miller said: "I guess all of us were in shock. . . . We didn't know what to do."
Miller said he started to leave even though O'Neill had threatened suicide. At the end of the driveway, he saw O'Neill's sister, Roisin, 21, who was returning from work at Maggie O'Neill's, a Drexel Hill pub the family used to own. He said he told her to call him.
When she asked what was wrong, he replied: "Go up the driveway and you'll see for yourself."
Roisin O'Neill testified she "broke down" after she saw the body, had a cigarette, and talked to Miller twice by cell phone before calling 911 - about a half-hour after she got home at about 3:10 a.m.
The repercussions of the shooting were amplified by the heart-wrenching testimony of Stacey Denancour, the mother of Sheridan's now 5-month-old baby.
Denancour, 18, of West Chester, described "the lonely journey" of her pregnancy and the painful challenge of single parenting.
"I wonder how I will ever explain this to my daughter," she testified.
Miller and Gallen were charged with reckless endangerment and related offenses for the vehicle-shooting spree. Gallen also faces a firearms charge. Later, on March 7, he was charged with resisting arrest and other offenses after he led police on a high-speed chase in Willistown.
In his memorandum, the prosecutor argued that the impact of the killing on the community, coupled with O'Neill's previous citations for underage drinking and marijuana possession, make him unsuitable for less than three years of juvenile supervision.
"Our courts give more supervision for stealing and using another's credit card," Conte wrote.
Of more concern, he wrote, was O'Neill's subsequent behavior at drinking party on St. Patrick's Day at the Penn Valley home of Rick Rueda, a Malvern Prep grad.
During an altercation with Jake Ramsay, 18, then a senior at Downingtown East High, O'Neill broke Ramsay's nose and grabbed a knife, witnesses said.
Conte pointed to an "arrogant and cold-hearted" comment attributed to O'Neill at the party: Both Ramsay and Leaghann Lambert, 18, a recent graduate of Bishop Shanahan High, testified that O'Neill said he had "killed better men for less."
Defense attorney Vincent P. DiFabio wrote that Ramsay saw O'Neill "sitting on a sofa with a knife in his hand, but the knife was never used in any threatening manner."
DiFabio, who suggested Ramsay initiated the fisticuffs, also referenced Lambert, who quoted O'Neill as saying: "I don't want to fight."
In his memorandum, DiFabio cited Mapes, the psychologist who found O'Neill overwrought with pain, guilt and depression from the shooting.
Mapes testified that O'Neill would benefit from substance-abuse treatment in the juvenile system. (He chose to pay a fine rather than seek treatment after his marijuana citation in February 2006.)
"The cases in which the appellate courts have agreed to allow the prosecution of a juvenile in adult court all seem to have a common factor: an intentional, violent felony," DiFabio argued.
In making his decision, the judge will have considerable discretion to weigh factors such as O'Neill's amenability to juvenile-treatment programs, his pre- and post-arrest conduct, and the impact of the offense on the community, said Monica D. Cliatt, a supervising attorney at the Widener University Civil Law Clinic in Harrisburg.
That discretion will make his ruling "very hard to overturn" on appeal, she added.
And although the defense has the burden of proving that O'Neill belongs in the juvenile system, Cliatt said the judge faces a greater burden.
"I don't envy him," Cliatt said. "He has to balance the competing interests of the community with what's best for this kid."
Contact staff writer Kathleen Brady Shea at 610-701-7625 or firstname.lastname@example.org.