A man smashed a bottle over the head of a fellow resident at a homeless unit in East Kilbride.
Robert Dow said he "snapped" because the man had been making life miserable for him.
His victim suffered serious injuries, including the loss of a piece of his ear.
Dow, 33, appeared from custody via videolink at Hamilton Sheriff Court on Friday.
He admitted assaulting the resident to his severe injury and permanent disfigurement in Kittoch Street on June 22 last year.
Dow was the subject of two bail orders at the time.
Chloe O'Hara, prosecuting, said the attack happened outside the Y Project homeless unit about 9.40pm.
She told the court a support worker saw the complainer walk past the front of the building covered in blood.
Ms O'Hara said: "A piece of his right ear was missing and he had a cut just above his ear."
The complainer said Dow had assaulted him, but when the police arrived he refused to say what had happened.
Officers found Dow nearby. He had cuts to his arm and wrist, and was taken to hospital for treatment before being detained by police.
His victim had a three-inch wound to his head and a one-inch cut to his ear.
The prosecutor added that the wounds were stitched, but the complainer then discharged himself against medical advice.
Police reviewed CCTV footage that showed the victim with two males, one of whom was the accused.
The accused was seen drinking from a bottle. He then moved out of view of the camera and a bottle smashed over the victim's head.
The bottle shattered because of the force and the victim ran off. Dow then walked back into view.
Defence agent Matthew McGovern said the victim had been "undermining" Dow in front of other residents.
The solicitor stated: "My client requested a transfer to another building, but this didn't happen. He accepts that on this particular day he snapped."
Mr McGovern said dad-of-three Dow has a "significant" record, but had stayed out of trouble for almost 10 years until 2020.
He added: "Unfortunately, a friend took his own life and another friend died from an overdose. He went off the rails and has had problems with alcohol and drugs."
Sheriff Thomas Millar noted that Dow has five previous convictions for assault, but accepted he had made efforts to change his lifestyle.
He jailed him for 18 months but since Dow has been in custody since his arrest in June he will be freed within two weeks.
The sheriff told him: "Hopefully, on your release you will continue to work against alcohol and drugs, and we won't see you back here again."
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