Mr Mitchell was stabbed on his way home from visiting his parents
A 17-year-old told police he stabbed another teenager because he was the "closest boy" to him, the High Court in Glasgow has heard.
Alan Dewar stabbed Joshua Mitchell, also 17, in an unprovoked attack as he walked home from his parents' house in Inverness in March.
He then stood smoking as he watched passers-by rush to help.
Dewar, who lived on the same street as the Mitchells, pleaded guilty to murder and will be sentenced next month.
On 22 March - the day of the murder - Dewar had been drinking and taking valium pills.
He had an argument with a friend on his way home and chased the other man to his house.
John Scullion, prosecuting, said Dewar - who had a knife in his pocket - tried unsuccessfully to get into the property.
As he stormed off, Mr Mitchell was walking home listening to his MP3 player after visiting his parents' house.
Mr Scullion told the court: "Without provocation, the accused approached him from behind and placed his left arm at shoulder level.
"An onlooker then saw the accused swing his right arm around to the front of the deceased and stab him once in the chest with the knife blade."
The court heard Dewar had told police: "I don't know why I done it. I don't know. I don't know why that boy. It was the closest boy to me."
Judge Lord Hodge told the accused: "You have pled guilty to the most serious crime.
"A wholly unprovoked murder and this is the type of tragedy caused by taking drink, drugs and carrying a knife."
Outside the court, Mr Mitchell's father Andrew paid tribute to his son calling him his "little angel".
The 40-year-old delivery driver also showed off a tattoo on his back which he had done in tribute to the boy.
Mr Mitchell said: "His mum Marilyn is lost without him. They were always together like peas in a pod.
"Joshua going has ripped the heart right out of me as, at the end of the day, he is not here anymore. His three brothers have also been badly affected.
"Joshua was never in trouble, he was a smashing lad. What happened to him was horrendous and utterly barbaric."
The court heard that Dewar had been jailed for possession of a knife seven months earlier and convicted of assault late last year.