There had been a restorative justice conference before yesterday's sentencing hearing but Ritchie's statements during the meeting were condemned by Judge Phillips.
The defendant had said there was "no time to p... around with cold water" on the day of the burning and that he had thought "the hot water might wake him up a wee bit".
"I find these comments extremely aggravating," the judge said.
A workplace accident at Lyttelton Port in 2013 had left Ritchie unable to work.
He would care for the 3-year-old victim while his then-partner worked on various farms.
On August 9, cracks in that arrangement began to widen when the child wet his pants.
Ritchie began yelling at the boy but his partner told him to stop.
In retaliation, the defendant pushed the child by the shoulder, causing him to fall over.
Ritchie grabbed his girlfriend by the chin and forced her up against a wall then to the floor before following her into the bathroom and placing his hands over her mouth and noseLess than three weeks later, at a West Otago property, Ritchie was supposed to be taking care of the boy.
The child damaged a poster on the wall, prompting Ritchie to smack him repeatedly, causing "substantial bruising" to the victim's buttocks.
That was a precursor to the violence that followed in the afternoon.
Judge Phillips said at 4pm, the boy had been left alone to watch a movie while Ritchie played video games in a bedroom.
"There was an incident with the child going to the toilet," the judge said.
"You became really, totally angry and purposely ran a hot bath. You were doing it for the purpose of punishing this child."
The results were immediate.
"The skin flaked off in the bath and continued to do so when he was removed by you," he said.
"One can only imagine the distress of this 3-year-old at the time."
Ritchie called the boy's mother immediately and applied cream to burns on the child's face and shoulders.
The wounds were so severe the child had to be flown to Dunedin Hospital.
Initially, Ritchie explained he was cooking some eggs and had boiled some water, which he accidentally spilled on the child.
Later he changed the story, saying he had run the bath but had tested the water and did not think it was too hot.
He also blamed the victim's bruises on him falling off a trailer when he was with his mother, before eventually coming clean about everything.
The child's mother said she felt "disgusted and hurt" by Ritchie's actions and Judge Phillips said it was hard to imagine a worse breach of trust.
"I consider the child to be totally and absolutely vulnerable," he said.
"This falls at the highest possible end of this type of criminal offending."
Ritchie was jailed for three years and 11 months.
