When an 11-month-old baby left in his care began crying, Diquan Lloyd Erwin Fisher shook her before she became limp and blue.
Fisher was 18 when the 17-year-old mother asked him to look after her sleeping baby girl in Brisbane's south in February 2020.
When she woke up and started crying after the mother had left, Fisher picked her up, shook her vigorously and told her to be quiet, the Supreme Court heard.
Fisher - now 21 - began "freaking out" when the child became unresponsive and the mother returned to the Corinda house.
He initially told the mother that the baby had been coughing, had vomited and been choking.
The baby girl was revived by paramedics at the scene but died at the Queensland Children's Hospital four days later, police said.
Fisher later conceded that he shook her from "side to side" when up to 40 "scattered bruises" were found on the baby that were consistent with where he had gripped the child, crown prosecutor David Nardone said.
The mother wept at Fisher's sentencing on Friday before leaving the courtroom in tears when details of the baby's death were revealed.
"Let me out. I need to get out," the mother said as she was led out by supporters.
She could be heard sobbing outside court, and was not able to return and read her heartbreaking victim impact statement.
"Finding out I was pregnant completely changed my life," she said in the statement that was read in court in her absence.
"Her love gave me hope for the future. I adored her with all my heart.
"She was always happy and giggling. She was loving, caring and would give me the cutest smiles and kisses."
The mother did not think she would ever recover from the loss of her "beautiful young woman", saying she struggled with anxiety and depression.
"It destroyed my life. How can anyone understand unless their first born was selfishly taken for no reason," she said.
"You took my beautiful young woman ... my world, my reason for living.
"The ... pain and heartache hasn't got any easier. I don't think it ever will.
"I would never have left her with him if I knew he could hurt her - I hope you get what you deserve."
Fisher was exposed to domestic violence at a young age, leading to his "desensitisation", his defence barrister said.
He began dabbling in drugs and alcohol to deal with anxiety and depression and "succumbed to ice", having a severe impact on his mental state.
Fisher also had "impulsivity issues" and struggled to maintain his composure, his defence barrister said.
Fisher had shown a great deal of remorse and guilt for the "catastrophic consequences" of his behaviour and for what he had done to the family, his defence said.
Fisher pleaded guilty to manslaughter on Friday and is expected to be sentenced by Justice Martin Burns next week.
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