Robert Hamill, a 25-year-old Catholic father of two was beaten by a gang of loyalists in Portadown town centre in 1997.
It is alleged by witnesses that an RUC Land Rover was parked in full view of the attack but that officers failed to act. But the RUC claim they did intervene but were driven back by the gang.
Mr Hamill, whose fiancee was pregnant at the time, died in hospital 12 days later.
Six men were charged with murder - but the case against five collapsed and the sixth was later acquitted due to a lack of evidence. Judge Cory said because of the difficulty in obtaining witness testimony, which may be due to fear of self-incrimination, a public inquiry may be more appropriate.
The judge expressed concern that at the time of the brawl in which Mr Hamill was killed, one of the officers failed to indicate in his early statements to investigators that he knew one of those involved. "This will become significant when examining the events giving rise to an alleged conspiracy to pervert the course of justice," Judge Cory said.
He also noted that in the trial of the five charged in connection with Mr Hamill's death that the presiding judge referred to a doubt over whether one of the RUC officers was telling the truth.
"It can be seen that the evidence presented to Lord Justice [name redacted] was both conflicting and worrisome," Judge Cory said.
He also noted some witnesses, such as other officers at the scene, had not been called and that their evidence may been of importance.
The RUC investigation also raised questions, Judge Cory found. "Why was there not a concerted effort made to track down those who witnessed the scene and might have identified participants?" he asked.