John Thomas "Cutthroat" Simpson pleaded guilty on Tuesday to reduced charges in the June 29, 2010, slaying of Bernard Hughes and wounding of Timothy Westfield at the British Wood Apartments.
Prosecutor Neal Pinkston said the 32-year-old Simpson agrees to testify against his three co-defendants - Unjolee Moore, Harold "Bam Bam" Butler III and Stephen "Kentucky" Ballou.
Simpson, appearing before Criminal Court Judge Don Poole, pleaded guilty to second-degree murder and attempted murder.
Three other charges against him were dismissed.
He faces 15-25 years at 100 percent on the second-degree murder and a concurrent 8-12 years on the attempted murder.
Prosecutor Pinkston said Simpson was the only one of the foursome who did not have a gun, and he said he also was not wearing a mask.
Simpson has been representing himself with attorney John Brooks as "elbow counsel."
The prosecutor said Moore earlier gave a statement in which he said "Cutthroat" had taken part. He later picked "Cutthroat" out of a police lineup as Simpson.
He said Simpson will be expected to testify in line with "a very detailed" statement he has given about what happened in the case. If he does not, he said the pleas will be set aside and he again will face a first-degree murder charge.
Prosecutor Pinkston said the foursome went to the residence of Hughes late in the night and asked to buy marijuana. He told them it was too late. There was then a struggle at the front door.
Hughes was found dead outside his apartment with multiple gunshot wounds.
Westfield survived despite two bullet wounds.
Simpson has been in custody for two and a half years.