Oscar Pistorius surrendered to emotion and let tears stream down his face on Thursday as a judge effectively cleared him of the murder of his girlfriend, Reeva Steenkamp.

Judge Thokozile Masipa did, however, appear to suggest Pistorius was guilty of manslaughter, known in South Africa as culpable homicide, by saying it was clear he had "acted unlawfully" in opening fire at his home on Valentine's Day last year.

If the culpable homicide verdict is confirmed – left as a possibility after the judge, just after lunch, abruptly adjourned the reading of her verdicts until Friday – Pistorius could receive anything from 15 years in prison to a suspended sentence, which would potentially allow the double amputee athlete, known as the Blade Runner, a chance to resurrect his sporting career.

After six months of fierce debate in court and beyond, Masipa accepted the paralympian's explanation that he mistook Steenkamp for an intruder when he shot four times through a locked bathroom door, killing her almost instantly. Pistorius sat in the dock and sobbed quietly as the judge ruled that the prosecution had failed to prove he was guilty of murder and the evidence was "purely circumstantial".

Pistorius being cleared of murder, in a trial that has riveted millions, surprised some in South African legal circles, and there were claims that Masipa had misinterpreted the law and sent a dangerous message.

Those inside the courtroom witnessed wildly swinging emotions and palpable tension. The Pistorius trial had become something of a television and Twitter soap opera in his home country and around the world. Now, after 41 days in court and thousands of pages of evidence, it was the turn of Masipa, only the second black female judge appointed to the high court, to take centre stage.

She entered just after 9.30am, walking slowly, bowing her head and taking her seat on the dais. She looked out at the now familiar wood-panelled courtroom: prosecutor Gerrie Nel to her left, defence counsel Barry Roux to her right, the teams of lawyers, Pistorius's brother Carl in a wheelchair after a car accident, Pistorius himself looking anxious in the dock, then his father and uncle and Steenkamp's parents on the front row and, beyond, journalists and onlookers. There was silence in courtroom save for the hum of an air conditioner, the flurry of fingers on laptops and the odd cough. Those in the room, and listening around the world, hung on every word.

Masipa effectively dismissed entire days' evidence with a stroke of her pen. Nel, who had become a minor celebrity, dubbed the "pitbull" for his aggressive style, saw large parts of his case demolished. Among them were intimate WhatsApp messages between Pistorius and Steenkamp, a model and law graduate, which dominated headlines but were deemed irrelevant.

Nel had gone to great pains to highlight messages that suggested acrimony, whereas the defence pointed to messages showing mutual affection. Masipa said they "prove nothing", adding: "Normal relationships are dynamic and unpredictable most of the time, while human beings are fickle. Neither the evidence of the loving relationship or a relationship turned sour can assist this court to determine whether the accused had the requisite intention to kill the deceased."

Neighbours who claimed they heard a woman screaming were similarly found to be "fallible" and unreliable. "Most witnesses had their facts wrong," she said. "I am of the view that they failed to separate what they knew personally or what they heard from other people or what they gathered from the media."

The judge dismissed these conflicting accounts and said the court had the advantage of technology: the record of phone calls that night. The timeline these established "tipped the balance" in favour of Pistorius's version of events, she said.

She was, however, damning in her assessment of Pistorius's testimony under cross-examination by Nel. "The accused was a very poor witness," she said. "The accused was, amongst other things, an evasive witness. He failed to listen properly to questions put to him under cross-examination, giving the impression that he was more worried by the impact his answers might cause rather than the questions asked."

Yet she observed that untruthfulness did not in itself mean the accused was guilty. She went on : "Viewed in its totality, the evidence failed to establish that the accused had the requisite intention to kill the deceased, let alone with premeditation. The state clearly has not proved beyond reasonable doubt that the accused is guilty of premeditated murder. There are just not enough facts to support such a finding."

Masipa dismissed claims that Pistorius shot because of a reflex action. "He took a conscious decision, he knew where he kept his firearm and he knew where his bathroom was. This is inconsistent with lack of criminal capacity. The intention to shoot, however, does not necessarily include the intention to kill."

Perhaps most controversially, Masipa ruled that Pistorius could not foresee that he would kill the person behind the door when he shot four times. Therefore he could not be found guilty of a lesser charge of murder. "How could the accused reasonably have foreseen that the shots he fired would kill the deceased?" she asked. "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 at the time."

At the end of the morning session, Pistorius sat silently for long minutes, engulfed in emotion, comforted by Carl and his sister, Aimee.

The ruling represented a victory for Roux, who had argued that the 27-year-old should have been tried for culpable homicide rather than murder because he had displayed an exaggerated "startle response" aggravated by his lifelong "disability and consequent vulnerability". Pistorius's lower legs were amputated when he was 11 months old. It will also bring some relief to his family, who were in court to support him every day, and the legion of fans who believed in him. It even raises the prospect that the Paralympian could one day return to the track.

But the judgment is likely to be condemned by friends and supporters of Steenkamp, including the African National Congress women's league, which was regularly represented in the public gallery. It will also fuel fears that Pistorius has received preferential treatment because of his wealth and fame.

Martin Hood, a Johannesburg-based attorney specialising in criminal firearms offences, said: "I'm disappointed and shocked. I think the judge has made some incorrect factual findings and applied the wrong legal tests. She should have found legal intent and found him guilty of murder. She's got it mixed up, unfortunately."

Hood claimed that every lawyer he had spoken to was in agreement. He added: "It sends out a very negative message that you can kill somebody, claim it was a mistake and get away with it. That not what we want in this country. We want to be hard on crime and intimate violence. It has created an extremely negative perception of the justice system."

Professor Rachel Jewkes, director of gender and health research unit at the South African Medical Research Council, blamed the law rather than the judge. "I think it is incredibly disappointing for women that the legal system is set up in a way that makes it extremely difficult to prove premeditated murder when there aren't other witnesses that could provide information. It's difficult to say a judge is wrong when she has to work within the parameters of the law. We're looking at a case where commonsense justice has not been done for Reeva and her family. The law doesn't always deliver what ordinary people call justice and that's clearly the case here."

Dean Peacock, executive director of Sonke Gender Justice, a group that campaigns against rape and domestic violence, said: "We respect the judge's decision and won't second guess it. In a country where the criminal justice system routinely fails gender-based violence victims, the verdict will easily be interpreted as evidence that violence against women goes unpunished. That's not Masipa's fault. That's the fault of an endemic failure to hold perpetrators of domestic violence and rape accountable."

Some observers felt the judge contradicted herself when, in finding Pistorius negligent, she said: "A reasonable person would have foreseen if he fired shots at the door, the person inside the toilet might be struck and might die as a result." This, according to critics, should constitute murder rather than culpable homicide.

Nevertheless, Pistorius returned to his uncle's home on Thursday night with a possible jail term still hanging over him. Masipa said a "reasonable person" would not have fired four shots into the toilet cubicle, and that Pistorius acted "too hastily and used excessive force‚ In the circumstances, it is clear his conduct was negligent."

With a culpable homicide conviction seemingly imminent, she then abruptly ended the day's proceedings in what many observers described as a "cliffhanger". Verdicts on this and three other firearms-related charges are expected on Friday, with sentencing to come some weeks later. Pistorius sat with his sister and aunt for a while then left the courtroom, declining to speak to journalists, passing through a melee of cameras and camera phones to what may be another night of unquiet sleep.