Dillon Hossbach admitted Friday that he killed his infant son last year by striking him in the head repeatedly.
"I struck him," Hossbach told Niagara County Judge Matthew J. Murphy III. "I hadn't had a lot of sleep the night before. I lost my temper."
Defense attorney Dominic Saraceno said Hossbach, 25, erupted because nearly 6-month-old William Hossbach, also called Liam, wouldn't stop crying.
Hossbach kept his composure Friday, answering questions in a calm voice.
Saraceno and First Assistant District Attorney Holly E. Sloma agreed in a private conference with Murphy last week that if Hossbach pleaded guilty to first-degree manslaughter Friday, he would be sentenced to somewhere between 12 and 15 years in state prison. He also will have to serve five years of post-release supervision.
The manslaughter charge carries a mandatory minimum of five years in prison, while the maximum is 25 years. But Hossbach was under indictment for second-degree murder.
"The murder (charge) would have been a possible life sentence," Saraceno said. "This was weighing on his conscience. No matter what the judge gives him, he gave himself a life sentence."
Sloma said the baby suffered at least six blows to the head, causing a subdural hematoma, or bleeding on the brain, which killed him.
William was reported not breathing and unresponsive in Hossbach's apartment on Rochester Road in Royalton about 11 a.m. Feb. 21, 2016, although Sloma said the attack actually might have occurred the night before.
Hossbach called 911 at 11:14 a.m.
His son, whose birthday was Aug. 22, 2015, was pronounced dead in Medina Memorial Hospital after more than two hours of attempts by paramedics to revive him.
The child's mother, Grace McPartlin, declined to speak to reporters. Murphy ordered Hossbach to avoid contact with her and with the couple's daughter, Charlotte Hossbach.
Murphy scheduled sentencing for July 14 and allowed Hossbach to remain free on $40,000 bail until then.