Confronted with the video, Covington confessed.
In a written statement to police, he said he had to kill McDermott because she was poisoning him with X-rays.
"I could feel the radiation when I went into the room," he said. "That's when I came to the conclusion that nobody would believe me about what she was doing to me."
It was an explanation that left McDermott's family members scratching their heads.
"To me it's a little ridiculous," said Angela Amarhanov, McDermott's daughter. "Honestly, it blows my mind that someone thought that my mom had a mean bone in her body, and that she would be capable of doing evil things."
Added Moran: "I hate to say that he's crazy. Because to me he was very calculating, and he had a gun, and he went up and he shot my sister in the head, very cold-bloodedly."
There were still more bombshells to come.
Before police closed the case on McDermott's murder, Covington admitted to having still more victims. He was no ordinary killer; he was a serial killer.
Two months before McDermott was gunned down, Covington shot and killed Odies Bosket, 36, at the Logan station of the Broad Street Subway line.
Bosket, a father of four, was on his way to pick up his 3-year-old daughter from nursery school.
In 1998, Covington murdered his cousin, the Rev. Thomas Lee Devlin. Devlin, 49, died in a hail of bullets as he was leading a prayer service in his sister's home.
In 2003, Covington jumped out from between two parked cars and shot David Stewart nine times as he walked home. Stewart, 43, miraculously survived.
In 2004, Covington also shot William Bryant, 33, as he walked to work. After shooting him several times from behind, Covington stood over the injured Bryant and fired two more shots. Like Stewart, Bryant was shot nine times and also survived.
Covington pleaded guilty but mentally ill to all these crimes, receiving a sentence of three life terms in prison and bringing to an end a one-man crime spree that spanned eight years.
"We are just glad that justice was brought to us and that he's not gonna be able to hurt anyone else," Amarhanov said outside the courthouse after Covington was sentenced in March.
Amarhanov now lives with Moran, who had promised her sister that she would take care of her children should anything happen to her.
"I know she'll be happy when she knows that we are doing so well or we are trying to do well," Amarhanov said.
Ironically, the video that showed her sister's murder also gave Moran some peace of mind, because it showed that her sister's last moments were not filled with fear.
"When I would watch it, I didn't sense that she was scared," she said. "She was just walking normally."
For law enforcement, the value of surveillance cameras could not be underscored enough.
Said district attorney Abraham: "McDermott's case might never have been solved. Who knows how many more victims there would have been had we not had that image of Covington murdering Ms. McDermott right on our video screens."