Nicholas Rogers, 28, stabbed Alex Stuart to death at a house party in the town’s Cuddyside, Peebles last August.
His terrified victim had sat cowering as burly Rogers armed himself with a knife amid claims he thought he had been laughed at.
He then plunged the blade into the 22-year-old’s chest second after stating: “I may as well kill somebody.”
Rogers, a former customer service worker, had denied murder during an emotional trial at the High Court in Glasgow.
He instead admitted killing Miss Stuart on the grounds his “ability to determine or control his conduct” was “substantially impaired” due to an “abnormality of mind”.
The court was told he had been diagnosed with a personality disorder. He had been deemed as a risk to others just days before the fatal attack.
But jurors rejected his special defence and convicted him of murder after a two-week trial.
Miss Stuart’s family sobbed heavily and consoled each other as the verdict was announced.
Supporters of Rogers also wept in the courtroom.
Rogers will learn the minimum term he will spend behind bars when he returns to the dock next month.
Judge Lord Summers told him to “be under no illusion” of the “gravity and seriousness” of what he had done.
The judge added: “The sentence will fully reflect the devastation caused to the family and the wider community.”
He also paid tribute to the “dignity” of Miss Stuart’s relatives who had sat each day listening to often distressing testimony.
Rogers repeatedly mouthed “sorry” to them as he was led handcuffed to the cells.
Miss Stuart was a postwoman and keen netball player with a “wide circle of friends”.
She had been at the house in Peebles with others when Rogers turned up swigging from a bottle of Edinburgh Gin.
The victim’s friend Rebecca Allen told jurors she did not get “good vibes” as Rogers started acting strange.
The 28-year-old was later in the living room with Miss Stuart, a sleeping Rogers and another man.
She and Miss Stuart were chatting on the phone to a friend and began laughing.
Rogers woke up and stomped into the kitchen. Jurors heard he came out clutching a knife with a black bag around the handle.
Miss Allen recalled: “He walked over and I said: “What is going on?’.
“He said: “You are not laughing now, are you?. I told him no one is laughing at you.”
He yelled he was a “evil b******”, a “hardman” and a “bad person”.
Miss Allen: “I was trying to reason with him, but it was not working. Alex was in total shock.
“She was sitting with her knees up, staring at me crying. He was scraping the knife at Alex’s legs, taunting her. It was like a power thing.”
Jurors heard he then held the knife “like a dagger” at his shoulder.
He then chillingly stated: “I have f****d it now – I may as well kill somebody.”
An emotional Miss Allen told the court: “I said there was no need for this. I kept saying that I wanted to get home for my wee girl.
“He said: “There is no point. You will tell on me. I am going to jail anyway.”
Miss Allen then remembered Rogers striking her friend Miss Stuart.
Describing the blow, she said: “He just went in a downwards action and stabbed Alex right in the chest.”
Brave Miss Allen tried to grapple with Rogers, but he smacked her head against a wall.
He then coolly walked out as Miss Allen rushed to try and save her friend.
She recalled: “I could see the colour fading from her face. I tried to keep her awake. There is nothing that I could have done.”
Miss Stuart died that day in Edinburgh Royal Infirmary.
Rogers went on to call his then lover Katrina Kelly stating: “I think I have killed someone.”
She went on to meet a ranting Rogers at the town’s Tesco supermarket.
He was soon held by police after Miss Kelly alerted a passing officer.
Miss Kelly said: “He was screaming at me that I had betrayed him for calling the police. I was a traitor.”
The killer went on to tell police: “Why did I do it? I am going to jail. I stabbed her. I don’t want her to die. I wish I had never done it.”
Miss Kelly had been aware of his mental health issues as well as his drinking and gambling problems.
She had contacted the local mental health team for help days before the killing as she believed Rogers was the “worst he had been”.
A psychiatric nurse met Rogers, who said he had been having suicidal thoughts.
Rogers was eventually marked under a heading “risk to others”.
His lawyers had urged jurors to convict him of the lesser charge of culpable homicide due to diminished responsibility.
But, the trial heard Rogers had “rejected” advice to stay off alcohol which could have an adverse effect on him.
Prosecutor Keith Stewart said, as a result, Rogers “behaved as he wanted with no heed for the consequences”.
Miss Stuart’s family were too upset to comment after the case.