A man feared he was going to die when a bodybuilder smashed a weight plate into his head.
Christopher O'Connor claimed he lashed out because he believed his victim was trying to break into a neighbour's home in the early hours of the morning.
The victim ended up covered in blood and with a fractured cheekbone.
Pharmacy manager O'Connor appeared at Hamilton Sheriff Court on Friday.
The 36-year-old admitted assaulting the man to his severe injury and permanent disfigurement in Leeward Circle, East Kilbride, on August 7 last year.
Alice Carey, prosecuting, said the victim had been out drinking with a friend. He was on his way home when he realised he had left his keys at the pal's house.
Ms Carey told the court: "He arrived in Leeward Circle about 1am and chapped on the door, but by this time his friend was asleep.
"O'Connor heard him and shouted at him to be quiet. The victim shouted back and O'Connor then left his house and ran towards him in possession of a 2.5kg weight plate.
"The victim ran backwards, but stumbled and fell. He tried to get up, but the accused pushed him down and struck him twice on the head with the weight plate.
"He tried again to get up and O'Connor hit him on the head once more. The victim then managed to run away to another house in the street.
"O'Connor followed and tried to apologise, but was asked to leave.
"Police officers arrived shortly afterwards and saw the victim's face was covered in blood and he appeared woozy."
The victim was taken to hospital where he received three stitches in a cut to his face. His cheekbone was fractured in several places and he had bruising to the chest.
The court heard that when O'Connor was asked by police officers what had happened he replied 'I was in fear for my life. I had to protect my property and my family'.
Defence agent Susan Gibson said O'Connor's conduct was "very much out of character".
She added: "He thought the complainer was a burglar as he was kicking his neighbour's door. My client shouted at him to calm down and go away but the complainer walked towards him, shouting 'You're fu**ing dead'.
"His position is that the complainer came towards him in an aggressive and threatening manner. He regrets his behaviour."
However, Sheriff Andrew McIntyre highlighted comments made by the victim in an impact statement.
The Sheriff said: "His perception was that the accused was trying to kill him.
"He describes in compelling terms the extent of it and his feeling that the accused was not going to stop - the accused being a big man and a bodybuilder."
Sentence on dad of one O'Connor was deferred until next month for background reports and he was allowed to remain on bail.
Sheriff McIntyre told him: "This is very serious.
"You haven't previously served a prison sentence and that is a distinct option, but I'll keep an open mind about the matter."
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