Oscar Pistorius has been cleared of the murder of his girlfriend, Reeva Steenkamp, a vindication for the athlete who vehemently protested his innocence during a trial in South Africa that gripped millions around the world.

But the judge abruptly adjourned the trial on Thursday afternoon with the culpable homicide – manslaughter – charge still outstanding. In delivering her ruling, judge Thokozile Masipa said it was clear Pistorius had acted unlawfully in shooting the person behind the toilet door on Valentine's Day last year.

She said a "reasonable person" would not have fired four shots into the toilet cubicle, and that Pistorius acted "too hastily and used excessive force … It is clear his conduct was negligent."

The court will resume on Friday to hear her decision on whether the athlete was guilty of culpable homicide, which could result in a prison sentence of up to 15 years.

The Olympic and Paralympic athlete slumped forward in the dock and sobbed quietly as the judge ruled that the state had failed to prove he was guilty of murder or premeditated murder, and that the evidence was "purely circumstantial".

The not guilty verdicts will 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 resurrect his career on the track.

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

Reeva Steenkamp. Photograph: Gallo Images/Getty Images

Dean Peacock, the 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."

After 41 days of testimony that ran to 352 pages of evidence, Masipa ultimately dismissed the prosecution case that Pistorius intended to kill when he shot four times through a toilet door at his home on the morning of 14 February 2013. Neighbours at his luxurious gated community in the capital of South Africa had testified how they heard "bloodcurdling screams" before and during a volley of shots.

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

Masipa said Pistorius was a "very poor witness", but that untruthfulness did not in itself mean the accused was guilty. She ultimately accepted his tearful insistence that he killed Steenkamp by accident after mistaking her for an intruder, and that he was traumatised by the death of the woman he loved.

"The state has not proved beyond reasonable doubt that the accused is guilty of premeditated murder," the judge said. "There are just not enough facts to support such a finding."

She said the accused believed his life was in danger and therefore could not be found guilty of a lesser charge of murder. "How could the accused reasonably have foreseen that the shot he fired would kill the deceased?" she said. "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."

The judge dismissed days of evidence about matters including police tampering with the crime scene and the length of extension cord in Pistorius's bedroom as ultimately irrelevant to the case.

At the end of the court session, Pistorius sat silently for long minutes, engulfed in emotion, comforted by his sister, Aimee, and brother, Carl, who is in a wheelchair after a serious car accident.

Henke Pistorius kisses the hand of his daughter Aimee as a judge reads her verdict in the murder trial of his son Oscar. Photograph: Reuters

The ruling represents a significant victory for defence counsel Barry 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.

The murder trial generated obsessive debate on Twitter and made legal history in South Africa by being broadcast live on television, giving the judicial system unprecedented exposure and offering a glimpse of a country still gripped by crime and the fear of crime.

Roux and prosecutor Gerrie Nel – who mauled Pistorius during cross-examination, confronting him with a photo of Steenkamp's bloodied head – became minor celebrities and several books on the case are in progress.

At the centre of the drama was Pistorius, a sombre and at times tormented figure in the dock and on the witness stand. Less than a year before the killing, the "Blade Runner" had been at the pinnacle of his career and set to make a personal fortune from brand sponsorships and endorsements. His agent, Peet van Zyl, told the court there were two stars of the 2012 London Olympics: Pistorius and the Jamaican sprinter Usain Bolt.

The South African's ascent had been the stuff of Roy of the Rovers comic books, overcoming his disability, bullying at school, an absent father and the death of his mother to run with the best in the world. Along with the Paralympic golds, he did charity work for children with disabilities and was seen as a role model and global brand. Van Zyl had predicted Pistorius and Steenkamp could be celebrities in the vein of David and Victoria Beckham.

All this lay in ruins just after 3am on Valentine's Day last year when Pistorius picked up his gun, walked to his bathroom and fired four lethally expanding 9mm bullets through the locked toilet door. On the other side of it was Steenkamp, a 29-year-old model and law graduate, who died almost instantly. Her mother was in court for most of the trial.

The mother of Reeva Steenkamp, June (2nd L), sits next to husband Barry, as she looks at Oscar Pistorius in court. Photograph: Phill Magakoe/AFP/Getty Images

Pistorius pleaded his innocence from the start, claiming he had been frightened by a noise in the bathroom and, assuming in the dark that Steenkamp was still in bed, had gone to confront an assumed burglar. During the trial, the athlete's defence appeared to shift under questioning from Nel. Having argued that the killing was a deliberate but mistaken act of self-preservation, he then said he pulled the trigger without thinking – an assertion that would match a so-called automaton or involuntary defence but not self-defence.

Nel, dubbed "the Pitbull", said Pistorius was an "appalling witness" who "dropped the baton of truth". Other observers described him as a poor witness who undermined his own credibility.

Yet Nel also admitted that the state's case was circumstantial. The defence produced a detailed timeline that it said demonstrated the screams heard by neighbours were not from Steenkamp but a desperate Pistorius, horrified and crying for help. It also highlighted police incompetence during the investigation including contamination of the scene and even the theft of Pistorius's designer watches.

The athlete's disability and sense of vulnerability may have weighed in the judge's mind as an explanation for his panicked response. Roux argued that Pistorius's double amputation had a cumulative effect on his psyche, much like a battered wife who kills her abusive husband when one final indignity pushes her over the edge. "You have to look at the slow-burn effect of that abuse over time," he said.

An acquittal had seemed unlikely to many. In March 2013, when he granted Pistorius bail, the magistrate Desmond Nair noted a number of "improbabilities" in the athlete's account of the shooting. Why, he asked, did Pistorius not find out who was in the toilet before opening fire. And why did Steenkamp not let him know she was there?