Alexander Pacteau, 21, admitted killing Miss Buckley when he appeared at the High Court in Glasgow. He will be sentenced next month.
An extensive search was launched for 24-year-old Miss Buckley when she was reported missing from the flat she shared with friends after failing to return home from a nightclub in Glasgow’s west end in April this year.
Her body was found four days later in a barrel at a farm on the outskirts of the city.
The court heard how Pacteau, who has a previous conviction for printing counterfeit £20 notes, had met Miss Buckley outside the club in the early hours of the morning and drove with her in his car to nearby Kelvin Way.
The car was parked on the street for 12 minutes, during which time Pacteau, who describes himself as a “self-employed sales consultant”, attacked and murdered her by grabbing her neck and delivering 12 or 13 blows with the spanner. Miss Buckley suffered injuries to her arm as she tried to defend herself.
Outside the court, Miss Buckley’s father, John, said his family remained “haunted” by the thought of the student fighting for her life.
He said: “All Karen was doing was making her way home when she was randomly targeted and murdered by a cowardly vicious criminal.
“No words of ours can do justice to our feelings towards him. He is truly evil and we hope that he will spend the rest of his life behind bars.
“Our hearts are broken at the thought of Karen’s final moments on this world. The thought of her being alone, frightened and struggling for her life haunts us. The panic and fear she experienced as she fought for her very survival but she had no chance against that coward.”
Miss Buckley, a nurse who was studying for a post-graduate qualification at Glasgow Caledonian University, was seen on CCTV leaving The Sanctuary nightclub in the early hours of Sunday 12 April. The footage captured her talking to Pacteau as they walked along Dumbarton Road.
In court yesterday, Lord Advocate Frank Mulholland described the lengths Pacteau had gone to cover his tracks, buying sodium hydroxide, or caustic soda, in an attempt to dispose of the body.
After failing to dissolve the body in his bathtub he took it to High Craigton Farm near Milgavnie, where police would find it in a plastic barrel padlocked in a storage unit.
Pacteau had initially told police that Miss Buckley had fallen and injured herself on his bed frame but he did not notice she had been bleeding until the following morning. He said that when he realised police were looking for information regarding her disappearance, he panicked.
He told them he had burned the mattress and clothes on a forest road because he was aware he was the last person to see her alive.
But the court heard how Pacteau, who sat in the dock with his head bowed, later changed his story, telling officers Miss Buckley had slapped him on the face when she was in his bedroom. He said he had grabbed the first thing to hand to hit her and she died.
“Of course that is not the case,” Mr Mulholland told the court.
“No words of mine can express the effect this terrible murder has had on the family,” the Lord Advocate said.
Pacteau has a previous conviction for printing counterfeit money and it has emerged that he stood trial at the High Court in Paisley in 2013 accused of attempting to rape a woman in a Glasgow lane. He was found not guilty of the charge.
Defence QC John Scullion told the court his client accepted “full responsibility” for his actions.
“He has instructed me to convey on his behalf an apology to Karen Buckley’s family and friends but he understands that such words are unlikely to give any comfort to them, and whilst I too recognise that nothing said on his behalf is likely to lessen the pain and suffering which he has caused to those who loved Karen Buckley, those are my instructions,” Mr Scullion said.
The lawyer said Pacteau could offer no “rational explanation” for his actions but had been “extremely intoxicated” when he met Miss Buckley outside the nightclub and had a “limited recollection” of what happened.
He said the pair spoke briefly in the street before driving off in his car in the direction of Miss Buckley’s flat. But shortly afterwards he said they had an argument over a “trivial” matter and he reacted angrily and lost his temper.
Mr Scullion said Pacteau accepted his actions in trying to dispose of Miss Buckley’s body were “despicable and beneath contempt”.
Judge Lady Rae deferred sentence until 8 September.
She told Pacteau: “This crime is a very shocking and disturbing case. You killed a young woman who was a stranger to you in what appears to be a motiveless, senseless, brutal attack.”
She added: “What you did after her killing, including telling the police a tissue of lies – some of which went into the public domain – would I have no doubt caused the family increased distress. All of that displays the actions of a man who is callous and calculated.”
Speaking outside court, Detective Superintendent Jim Kerr said there was no connection between Pacteau and Miss Buckley and that it had been a “random attack”.
He said: “It could have happened to any female that night, we can’t see anything from our investigation that would have predicted what he did on the night in question.”
Asked if Pacteau was waiting outside to prey on a woman, Mr Kerr said: “I think there was premeditated plan that night to find some victim, yes I do.”