Devout Muslim Bilal Abdulla told the man to “stop playing and start praying” after taking offence at the instrument, a jury was told.
Woolwich Crown Court heard evidence of an intimidating exchange at his Cambridge flat in 2005 from former friend Shiraz Maher.
Mr Maher said the two students were working at their computers when a third man started to play a guitar in another room.
Abdulla gestured towards a violent video of an ambush of an American Humvee vehicle in Iraq on his computer.
Mr Maher said: “He made reference to one of those grisly, very grisly videos that would come out of Iraq at that time.
“He said, ’I told him, you need to stop playing and start praying otherwise this is what we do - we slaughter.”’
He added: “Immediately after saying it Bilal laughed heartily and I remember laughing as well.
“The reason it stayed so vividly in my memory was the degree to which it was disproportionate.”
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