"Did you mean for any of this to happen?" defense attorney James A. Funt asked.
"Not at all," Shaffer sobbed. "This should never have happened."
Shaffer is on trial for third-degree murder and conspiracy in the fatal attack on Wallace on April 8, 2010, at Knights and Fairdale Roads in the Northeast.
Shaffer's father, Gerard Sr., a 23-year city firefighter, was also charged but died at home of a heart attack Dec. 5, 2011, at age 48.
Prosecution and defense lawyers completed their cases Thursday after three days of testimony.
On Friday, the jury of seven women and five men will return to the Criminal Justice Center to listen to closing arguments from Funt and Assistant District Attorney Peter Lim.
Jurors will then get instructions in the law from Judge Lillian Ransom and begin deliberating.
Lim has argued that Shaffer willingly took part in the fight to show his father he was "a tough guy."
Shaffer's lawyers have described him as a victim caught between a mercurial father who beat him and his brother, and Wallace, an alcoholic who defense lawyers said became violent when intoxicated.
"How many times did your father hit you?" Funt asked Shaffer.
"I couldn't tell you," Shaffer said.
"How about your brother?" Funt asked.
"Too many times," Shaffer replied.
Shaffer's older brother, Ernie, committed suicide several years before the 2010 encounter.
The confrontation that ended with Wallace's death began about 7:30 p.m. on April 8, 2010. Witnesses testified that Wallace was crossing the intersection and walked in front of Shaffer Sr.'s gold Dodge Durango, stopped eastbound on Fairdale at the light.
Shaffer began honking his horn and swearing at Wallace to get out of the way; Wallace yelled back, "I have the right of way."
Witnesses said Shaffer Sr. parked the SUV, got out, and pursued Wallace on foot north on Knights, and confronted him at the entrance to a gas station.
Wallace held his hands up and backed away, witnesses testified, repeating that he "had the right of way." Shaffer pushed Wallace in the chest and knocked him back several steps, witnesses said. Wallace approached and threw a punch, and the pair started fighting, they said.
Shaffer Jr. got out of the SUV and ran to the fight, calling after his father, witnesses said. They said the son pushed his father out of the way, grabbed Wallace in a bear hug, and flipped him over his shoulder headfirst onto the concrete.
Both men got back into the SUV and drove off, witnesses said. Wallace died of massive brain injuries two weeks later.
Questioned by Lim, Shaffer corroborated the witnesses' version of events with two exceptions.
He insisted that after he grabbed Wallace, both of them fell to the ground and Wallace must have struck his head.
Shaffer also rejected the account of a witness who said she saw him punch Wallace in the face.
"So the witnesses are right about everything else but wrong about that?" countered Lim.
"Yes, they're wrong," Shaffer said.
Lim also pressed Shaffer about why, if Wallace's death was accidental, he did not try to help or call 911 on his cellphone.
Shaffer said he was scared and his father ordered him back to the SUV.
He wept as he recalled his father, telling the jury, "I loved him, but he had a hot head. He had a bad temper."
Shaffer said he watched his father drag Wallace's body to the curb and walk back to the parked SUV.
"He got to the car," Shaffer said, "and he said, 'This is going to be rough, but we're going to get through this.' "
Contact Joseph A. Slobodzian at 215-854-2985, firstname.lastname@example.org, or follow @joeslobo on Twitter.