Josh Hanson's throat was slit by a man in a pub in an act of "pitiless savagery", a court has heard.
Shane O'Brien, 31, fatally stabbed Kingsbury man Josh after a brief conversation in RE Bar in Hillingdon, west London, in October 2015 then went on the run for more than three years, the Old Bailey heard.
Prosecuting, Mark Heywood QC told jurors what took place was "sudden and shocking".
O'Brien and Mr Hanson had never met before their paths crossed for a matter of seconds in the bar, jurors were told.
Mr Heywood said: "For reasons that have yet to be fully explained, the defendant stood up and approached the other man.
"As they spoke briefly, with others around them, the defendant reached for his blade and with a single, slashing motion, he used it. He cut his throat.
"You will judge for yourselves when you have heard the evidence but it was an act of pitiless savagery."
The prosecutor told jurors O'Brien accepted he used the blade to cause Mr Hanson's death but says he is not "criminally responsible" because he did not mean for it to happen.
Earlier on Saturday October 10, 2015, Mr Hanson had gone to hospital with his girlfriend after her father suffered a suspected heart attack.
They then met up with his cousin and a small group of friends for a drink in RE Bar.
About an hour later, O'Brien arrived with three other men, the court heard.
Just after 1am the defendant allegedly approached council worker Josh and, standing in front of him, asked: "What's your problem?"
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Mr Hanson appeared confused but did not react or challenge the defendant, jurors heard.
O'Brien was shown in CCTV footage to "surreptitiously" reach for his blade in the pocket of his expensive Canada Goose padded jacket, the court heard.
Mr Heywood said: "This part of it took less than 10 seconds as the CCTV shows. Five to six seconds after he had first reached for it [the blade], the defendant struck. He suddenly raised his right hand, holding the blade, to a level above the head of Josh Hanson.
"He then brought the blade down in a sweeping, slashing motion, aimed at and striking the exposed skin of John Hanson's ear, neck and chest.
"It was a deliberate, swiping motion, intended to cause, at the very least of it, very serious injury to Josh Hanson with that razor-edged blade - aimed at the vital structures of the neck."
The jury was shown graphic footage of the moment Mr Hanson was attacked in front of his girlfriend.
As blood immediately flowed from a large gash to the neck, O'Brien walked out of the bar, the court heard.
Mr Heywood said the stabbing was "catastrophic" and left a 37cm long gaping incised wound from the victim's left ear to right chest.
Before he could be apprehended, O'Brien was flown in a private plane from Biggin Hill airport to the Netherlands, the court heard.
Despite an international manhunt, O'Brien evaded authorities for years, jurors were told.
He allegedly used various fake identity documents, including a Danish passport, and got a new tattoo.
He was finally arrested in Romania on March 23 and brought back to Britain on April 5.