The fact that Judge Thokozile Masipa did not find Oscar Pistorius guilty of murder, specifically the murder of an intruder, was "puzzling," a legal expert said on Thursday.

"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," Pierre de Vos wrote in his blog, Constitutionally Speaking.

Masipa started handing down judgment in the High Court in Pretoria on Thursday.

The paralympic athlete shot and killed his girlfriend Reeva Steenkamp behind a locked toilet door at his Pretoria home on Valentine's Day last year. He said he mistook her for an intruder.

Masipa found that Pistorius could not be found guilty of murder, as he lacked the direct intention or indirect intention (dolus eventualis) to kill the person he believed was hiding behind the toilet door.

However, De Vos said Pistorius could have foreseen the possibility of killing the person behind the door, whether it was Steenkamp or an intruder.

"The State can only prove intention via the concept of dolus eventualis where the State can prove that while Pistorius might not have meant to kill the victim (Reeva Steenkamp or the putative intruder), he nevertheless foresaw the possibility and nevertheless proceeded with his actions," said De Vos.

"To my mind the judge did not engage with this issue in sufficient detail to explain convincingly why she found that Pistorius did not have the dolus eventualis to kill an unknown person behind the toilet door."

De Vos said if Pistorius had intended to kill an intruder (and not Steenkamp), he would still be guilty of murder as long as the State had proved beyond reasonable doubt that he had intended to kill the person behind the door whom he might (or might not) have thought to be an intruder.

"In South African law it is not a valid defence to claim that you did not have the intention to kill X because you had in fact intended to kill Y and had killed X by mistake."

However, De Vos believed that the State failed to produce sufficient evidence to prove beyond reasonable doubt that Pistorius intended to shoot and kill Steenkamp, as their case was "almost exclusively circumstantial" and they failed to produce sufficient evidence to prove beyond reasonable doubt that Pistorius intended to shoot and kill Reeva Steenkamp.

"It would be difficult to convict somebody for murdering his girlfriend merely because the State produced evidence that they had fought on Whatsapp and (contradictory) evidence that neighbours heard them fighting on the night of the killing," he said.