Oscar Pistorius was convicted of culpable homicide by Judge Thokozile Masipa after being cleared of murdering Reeva Steenkamp.
The prosecution accused Pistorius of premeditated murder, claiming he had deliberately shot his girlfriend Steenkamp after an argument on Valentine's Day last year.
However, Judge Masipa told the court that the state had failed to prove "beyond reasonable doubt" that Pistorius is guilty of premeditated murder. "There are just not enough facts to support such a finding," she said.
Masipa said the evidence the state offered on the charge was "purely circumstantial".
Based on the objective facts, such as phone records, she accepted the defence's timeline of events that the shots were fired at around 3.12am. This meant that some of the state witnesses who claimed they heard a woman screaming after the time Steenkamp was shot must have been "genuinely mistaken", she said.
The judge also said that the WhatsApp messages between Pistorius and Steenkamp did not "prove anything" and the evidence suggesting Steenkamp had eaten two hours before she died was "inconclusive".
Masipa then turned to the lesser charge of murder. She said there was "no doubt" that when Pistorius fired shots at the door he "acted unlawfully".
However, she said that the evidence does not support the state's case that this was "murder dolus eventualis", a legal term for when the perpetrator foresees the possibility of his action causing death and persists regardless.
Masipa accepted that Pistorius believed Steenkamp was in the bedroom, noting that this part of his account had remained consistent since the moments after the shooting. It is "highly improbable the accused would have made this up so quickly", she said.
She described Pistorius as a "very poor" and "evasive" witness, but said it did not mean he was necessarily guilty.
"Clearly he did not subjectively foresee this as a possibility that he would kill the person behind the door – let alone the deceased – as he thought she was in the bedroom," she said.
Legal experts suggested that the state might be able to appeal the murder ruling. Masipa explained why Pistorius did not foresee that he would kill Steenkamp, but did not "explain convincingly" why she believed he did not foresee that he would have killed the perceived intruder, says Pierre De Vos, who teaches constitutional law at the University of Cape Town.
Writing in South Africa's Daily Maverick, De Vos says: "Given all the evidence presented in court about Pistorius's knowledge of guns and what the bullets he used would do to a person, it is unlikely in the extreme that Pistorius did not foresee that the person behind the door (who he might have thought was an intruder) would be killed."
Masipa offered a legal explanation as to why she could only convict Pistorius on culpable homicide rather than murder.
A "reasonable" person with Pistorius's disabilities would have foreseen that shooting into the door may have killed the person inside, she said.However, South African law warns against automatically assuming that because a perpetrator "should have" foreseen the consequences of his actions that he actually did.
She pointed to JM Burchell's General Principles of Criminal Law, which states that "the courts have warned against any tendency to draw the inference of objective foresight too easily".
Following previous cases, the courts have been told to "guard against proceeding too readily from 'ought to have foreseen' to 'must have foreseen'".
The onus was on the state to prove beyond reasonable doubt that Pistorius foresaw the fatal consequences of his actions when he shot at the door. Masipa said the prosecution failed to do so.
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