The man and woman heard the commotion breaking out in their otherwise sleepy small town neighbourhood in the early hours of Saturday morning.
The pair approached the 33-year-old man who was on Monday remanded into custody for double murder, seeking to intervene after realizing from the screaming and shouting that it was no ordinary argument.
"I wondered what the hell he was doing. I thought at first he was standing there and hitting the hill," the boyfriend of the 57-year-old woman told the Aftonbladet daily, explaining that he realized they were watching a boy being beaten with a metal rod.
The man told the newspaper that the couple then ran towards the offender, screaming at him to stop.
"He came at us like a madman. He was only a few feet away from us when I screamed."
The 57-year-old woman sustained a fatal blow in the ensuing scuffle.
"I managed to push him over and a man who was coming over the bridge came over to help to hold him down. I screamed that he should calm down."
The man later managed to flee and was later arrested not far from the scene of the crime. The 33-year-old has been charged with double murder and was on Monday remanded into custody at Linköping District Court.
The man is reported to have a history of mental illness and has an extensive criminal record.
Prosecutor Britt Louise Viklund has indicated that the police investigation extends to further suspected offences in connection with the incident, aside from the two murders.
The man's lawyer, former justice minister Thomas Bodström, has stated that the perpetrator denies the charges and claims that he has no memory of the events in question.
At 2am on Saturday morning a woman alerted police that a man had broke into the basement of her house. The man was accompanied by two boys, born in 1998 and 1999, who he is reported to have abducted after following them home from a youth centre. The woman ended up in a scuffle with the man and her boyfriend arrived to help, managing to secure the release of one of the boys.
The man is then believed to have left the house with the older boy and was challenged by the 57-year-old woman and her boyfriend. The woman and the boy were killed in the ensuing scuffle and the man was left nursing injuries.