Student Constable Sipho Mbatha was convicted of the murder of Soweto teenager Thato Mokoka by the High Court in Johannesburg on Friday.
Judge Kathleen Satchwell said there was no evidence that disputed his act was not intentional.
"The barrel of the gun wasn't pointed at his feet or legs. The gun was pointed at his head and back. Any shooting would have resulted in death, not injury," she said.
"I heard no evidence that suggests Mokoka moved and accidentally caused the shooting. There's no evidence that suggests any of your clothing or uniform perhaps (latched) onto the trigger... ."
There was also no evidence suggesting the gun was not properly handled, thus leading to the shooting.
Satchwell said the fact Mbatha chose not to testify had an impact on the trial.
"I deeply regret that you did not give evidence."
Sipho Mbatha shot dead Mokoka, 16, outside his grandmother's house in Braamfischerville, Soweto, on February 14.
Police had been responding to a complaint that Mokoka was part of a gang and owned a firearm.
The firearm in question was never found.
Sentencing proceedings continue.