A three-year-old boy with suspected autism suffered fatal head injuries in a complete breach of trust by his killer, a lawyer told a court.
Kayden McGuinness sustained 15 bruises on the scalp and died from swelling and bleeding of the brain.
He was found dead in bed at his family flat in Londonderry in September 2017.
Liam Whoriskey, 25, from Londonderry, was convicted of manslaughter and cruelty to a child by a jury at the city’s crown court in October.
The former partner of the vulnerable toddler’s mother was supposed to be fulfilling the role of parent, the court heard.
Prosecutor Peter Irvine QC said: “There was a complete breach of trust in this case.”
The child died from blunt force trauma.
Probation officers said Whoriskey, from Glenabbey Gardens, posed a significant risk to other members of the public.
His former partner Erin McLaughlin said he was aggressive and violent.
She said: “He went from 0-60 – roaring and shouting.”
A pre-sentence hearing was held at Londonderry Crown Court on Friday.
Kayden was found to have “extensive bruising” on the face and 15 bruises on the scalp caused by “blunt force trauma” to the head.
That caused haemorrhage and swelling of the brain, Mr Irvine said.
He died within half an hour from “non-accidental” injuries, examinations showed.
Whoriskey had earlier taken the youngster to the city’s Richmond shopping centre.
The child was seen lying on the floor of one of the shops and an assistant described Whoriskey’s behaviour as “abusive and violent”, the lawyer told the court.
He said the defendant was observed holding the child by the hood of a jacket and by the wrists.
Kayden was struggling to keep up with him while walking, he added.
Earlier in the year his mother, Ms McLaughlin, had observed a mark across his nose.
Experts who gave evidence to the court testified that he had two black eyes caused by at least moderate force, the prosecution said.
Mr Irvine said: “This is a highly exceptional and very serious case where the culpability of the defendant is extremely high – the deceased was clearly very vulnerable, a three-year-old boy.”
He said pre-sentence reports showed the defendant posed a significant risk of serious harm.
The defence challenged the term blunt-force trauma.
Ciaran Mallon QC said expert witnesses had suggested the gravity of the conduct causing the injuries could be viewed as mild, minor or, at its height, moderate.
He said culpability was not extremely high and his client had not intended to kill the child, rejecting any suggestion a life sentence was appropriate.
He pointed out that the child’s skull was not fractured and said sentencing severity should not approach the “heights” which the prosecution had sought to set out.
Whoriskey will be sentenced by judge Philip Babington on December 16.