Christopher Johnson told Preston Crown Court how moments before the fatal wounds were inflicted, two "angry and aggressive" neighbours fought with him and victim Adam Le Roi at the block of flats they all lived at, after they confronted the men over nuisance behaviour involving a man running across a roof.
Mr Le Roi, 25, who lived on the third floor, was stabbed multiple times at Kayley House on New Hall Lane in Preston on Sunday, November 15, and died in the Royal Preston Hospital.
UCLan archaeology graduate Connor Rumble, 25, who lived on the first floor, denies his murder alongside fellow graduate Grant Gardner, 24, who is alleged to have kicked Mr Le Roi repeatedly as he lay on the ground, moments after the injuries were caused.
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Giving evidence for the prosecution, Mr Johnson was visibly distressed as he was asked to watch CCTV footage of the incident.
The court was told prior to the attack, Mr Johnson was unhappy because he had seen a man he believed to be from the defendants' flat running across a corrugated roof.
He said: "I proceeded to go downstairs as I wasn't happy. I have family in that block of flats who had a newborn baby at the time so I proceeded to go down to resolve the situation and ask why they were on the roof.
"I did go alone at first and Adam followed.
"There was only one flat to that side at that end. It had to be that flat."
He said the first two men who came to the door were aggressive, but their two other friends were trying to resolve the situation.
Later in the evening Mr Johnson saw a man out on the roof again and proceeded to go back down to knock at the flat again, followed by Mr Le Roi.
He was asked by prosecutor Louise Blackwell QC to describe what had happened and replied: ""The two "angry and aggressive males" opened the door.
"Adam was punched by one of the males, with his right hand, in his face.
"Adam reacted and then proceeded to punch the man who punched him in the face, he punched him back.
"Then the other male proceeded to pull Adam's head down - that's when I reacted and started throwing punches left right and centre.
"I then proceeded to start punching the male with the white T shirt until I put him on the floor
"After that I pulled him out of the door and then we had a scuffle in the corridor."
Mr Johnson said he pulled Mr Le Roi off the taller man and pulled him outside of the door and they walked towards the lift.
He added: "The male in the white T-shirt then came out frantically with a knife in his right hand
"I'd describe it as a black handle with a six to eight inch blade.
"He was frantically throwing it back and forth above his shoulder. He was screaming: 'I'm going to f***ing kill you.'
"I then proceeded to walk through the fire door as I was in a panic and quite scared. Out of my peripheral vision I saw Adam walk towards the male in the white T-shirt.
"I just briefly remember turning my head, walking back to the fire door and I saw Adam on the floor.
"I proceeded to pick Adam up and I grabbed him and pulled him through the door.
"The male with the white T shirt was stood at the door waving the knife around still and I put my foot behind the door so he couldn't come back through it.
"Adam had got in the lfit and I saw a blood stain on the left hand side of the entrance to the lift."
He described taking the stairs to get to the flat, then realising Mr Le Roi was not there.
He added: "It's very hazy.
"I asked my partner where Adam was."
He began to cry as he added: "Then I went back to the lift and found Adam.
"I just opened the lift door. "
He told the court he was "face down in a pool of blood" and that he could not see where blood was coming from.
He flipped him over and saw he was spitting out blood, and called 999.
He said: "I can't remember which side but it was pouring out of one side, and the lift floor - it just turned red.
"I didn't get a response."
" I was very distressed, extremely upset."
Medics gave him instructions to carry out CPR and use his clothes to "plug" any wounds in his chest to stem the bleeding.
"A friend of Mr Johnson and his partner then arrived and took over doing CPR.
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