Dominic O’Connor, aged 28, faces up to five years in prison. He used a lead to kill the animal in the North in December 2016.
He cooked its body and used it as pet food for his other dog, Shadow.
His barrister, Chris Holmes, said: “Clearly, this is an unusual and extremely disturbing case.”
The father of two has no memory of the incident, which occurred at his Roden St home in the Co Down village of Kircubbin, his lawyer told Downpatrick Crown Court. O’Connor was convicted by a jury last month.
He bought the dog on Gumtree from a couple in Lisburn, Co Down, who have not been traced, the court was told.
The judge said O’Connor strangled it with a lead, stripped it of its skin, “butchered” it, cooked it, and fed it to his other dog.
The other animal has been rehomed and the judge said that social services should be informed of O’Connor’s conviction. Mr Holmes, referring to a medical report, said O’Connor “exhibited quite disturbing personal symptoms, leading to a psychopathic disorder”. He said his client lacked empathy.
Consultant psychiatrist Ian Bownes examined the defendant.
Mr Holmes added: “This is bizarre and the motivation behind it is, basically, inexplicable.”
Judge Piers Grant challenged the lawyer’s authority to make those observations, but said much of the evidence came from O’Connor’s own lips and was clear.
The defence said O’Connor was adopted at the age of four, attended third-level education, and was employed as a mental health nurse, “which is deeply ironic, in this case”, Mr Holmes added.
O’Connor has previous convictions, for possession of an offensive weapon and for common assault.
His position “deteriorated comprehensively” in 2015, following the death of his mother and his marriage break-up, his lawyer said.
His lawyer said there was a “complex and disturbing” mental health background in this case.
Dr Bownes’ report suggested he had a disordered personality of the psychopathic type, Mr Holmes disclosed, and a troubled background.
The court was told it was an extremely serious and distressing case.
Prosecution lawyer, Laura Ivers, said the sentence imposed should be the highest possible of the five-year imprisonment maximum.
The judge observed that, in some cases, there could be no explanation for wrongdoing.
He adjourned the case for sentencing on November 29.