RCMP Const. Scot Newberry "used excessive force during the course of an unlawful arrest," Chief Judge Brian Bruser said in a written decision.
"(The victim) was enjoying the protection afforded by our laws and while under the umbrella of the rule of law, Constable Newberry attacked him."
Bruser made the comments March 4 while dismissing a pair of charges against 23-year-old Devon Herback.
Herback faced one count each of assaulting a peace officer and obstruction of justice following an Oct. 30, 2004 confrontation with Newberry.
According to testimony from the trial, Herback left a Yellowknife nightspot with his girlfriend, shortly before closing time. The couple argued on the street and Herback testified his girlfriend slapped him once on the face.
Newberry and Municipal Enforcement Officer Dana Jones, who was also in the police vehicle, testified Herback grabbed his girlfriend by the hands, prompting the officers to pull over and investigate.
While the testimony from that point forward differed, Bruser agreed with Herback who said the officer asked him if he "enjoyed" beating his girlfriend.
Herback became agitated and began to swear - at which point Bruser said Newberry punched him in the temple with "considerable force."
The blow knocked Herback unconscious. He fell to the ground and broke his leg.
"It was the officer who escalated the hostilities, Bruser wrote. "(Herback) was not looking for a fight with police. All he wanted to do was go home."
The 220-pound Newberry testified he was simply trying to restrain the 145-pound Herback, who Newberry said tried to kick him in the leg.
Under cross-examination, Newberry repeated several times that his memory of the incident was incomplete. Bruser called his testimony "frequently vague, evasive, implausible or simply unbelievable or unreliable."
RCMP Const. Kerri Riehl said police were aware of the ruling and consider the matter "an internal quality control issue."
While members of the Yellowknife detachment are examining the allegations, Riehl said the case has not reached the stage of a full-blown internal investigation. "We will review the judge's decision in detail... before we make any determination," Riehl said.
"We take any question about the quality of our work seriously."
While Riehl could not specifically discuss Newberry's employment status because of privacy concerns, she said no officers have been suspended within the last month.
Acting head Crown prosecutor Sadie Bond declined to comment on the case publicly, but said any investigation into possible assault charges would be the responsibility of the RCMP.
Attempts to contact Herback through his lawyer were unsuccessful.