The racist texts found on the phones of two of the three G4S security guards who escorted Angolan deportee Jimmy Mubenga to his death in 2009 required a double take. One text, written by defendant Stuart Tribelnig, read: “Fuck off and go home you free-loading, benefit-grabbing, kid-producing, violent, non-English speaking cocksuckers and take those hairy-faced, sandal-wearing, bomb-making, goat-fucking, smelly raghead bastards with you”. Meanwhile, 76 racist texts were found on the phone of G4S guard Terrence Hughes, which were targeted at black Africans, Asians and Muslims.

Ultimately, the judge, Mr Justice Spencer, decided the texts were not relevant to the prosecution of the guards, and the jury subsequently ruled that they were not guilty of manslaughter, after forcing Mubenga’s head down and restricting his breathing as the flight prepared to take off at Heathrow airport. More than 20 people had heard Mubenga say over and over “I can’t breathe.”

Whether the judge was right is another matter. But it does leave us with a number of unanswered questions. What kind of company would employ such individuals? What checks were done before they were employed? And how can transnational companies such as G4S be held accountable for their employees’ actions?

It is tempting to believe that the Mubenga case is a one-off. The reality is very different. While G4S employs many guards who do a professional job and abide by the terms of their contracts, it has a disturbing record of employing people with a history of racism, violence and/or criminality. It has an equally disturbing history of employing individuals who kill while in their employ, or restrain people in a manner that results in death.

Last October, G4S security guard Clive Carter was jailed for life after killing female conference delegate Khanokporn Satjawat at Glasgow’s SECC conference – he followed her into the ladies toilet and bludgeoned her to death with a fire extinguisher because she complained about him using her security pass. Every bone on the left side of her face and neck was broken and her skull was shattered. Carter had a record of getting in a rage with women who contradicted him.

This June, G4S was also accused of violently removing protesters from its own AGM at London’s ExCeL Centre (an allegation the company has denied). A couple of months ago G4S, alongside Serco and the Youth Justice Board, had to pay out almost £100,000 for unlawfully restraining youngsters in secure training centres.

In 2011, double amputee Palaniappan Thevarayan died when his unsecured wheelchair tipped over backwards as he was being transported to hospital in a G4S ambulance to St Helier hospital in Surrey. Last year’s inquest found that the driver and G4S staff had not received sufficient training to move patients safely between their homes, hospitals and clinics.

Earlier this year we spoke to the family of Danny Fitzsimons, a former soldier who was employed by G4S in 2009 when he had post-traumatic stress disorder, a criminal record and a history of racism, and was on bail and not allowed out of Britain. Within 36 hours of arriving in Iraq to work for G4S subsidiary ArmorGroup he had shot dead two colleagues, Scottish security guard Paul McGuigan and Australian Darren Hoare. His stepmother, Liz Fitsimons, told us she blamed G4S for both the murders and the fate of her son – if G4S had checked his records it would not have employed him. G4S told the Guardian it accepted that on that occasion: “His screening was not completed in line with the company’s procedures.”

In 2004, 15-year-old Gareth Myatt, who was mixed race, died after being restrained by three officers in Rainsbrook Secure Training Centre. Myatt was 1.47m (4ft 10in) and weighed 41.3kg (6st 7lb). One of the restraining officers, David Beadnall, was 1.85m (6ft 1in) and 101.6kg (16st). When Myatt told the restraining officers he could not breathe, one replied: “Well, if you are shouting, you can breathe.” No officers were charged.

The inquest found that Myatt’s death was an accident, but it also concluded that Myatt might still be alive if the Youth Justice Board, which oversees privately run centres like Rainsbrook, had carried out adequate safety checks into the types of restraints G4S staff were using on the children in their care. Beadnall was subsequently promoted to safety, health and environmental manager at G4S Children’s Services. The female officer involved in the restraint, Diane Smith, tried to get damages for PTSD after the incident. Her claim was rejected in the high court and she went to the appeal court. Her claim was again dismissed.

These controversial incidents are by no means restricted to the UK. Earlier this year, G4S confirmed that company staff were involved in violent riots at the Manus Island detention centre in Papua New Guinea when Iranian asylum seeker Reza Barati was beaten to death. In 2009, an Australian coroner blamed the company for the heat-related death of a 46-year-old Aboriginal man who “cooked to death” in the G4S prison van transporting him.

Racist practice by G4S employees has been frequently exposed – but to little effect. A 2007 report by the charity War on Want claimed that some black employees in South Africa had been forced to use different toilets to white employees, and that white supervisors had referred to them as “kaffirs” and “monkeys”. It also said that G4S, which had a turnover in excess of £4bn that year, paid some black staff “poverty” wages. (G4S denied the accusations, saying allegations of racism are thoroughly investigated and insisting it paid its African workers far higher than the minimum wage in the respective countries.)

In 2011, the chief inspector of prisons, Nick Hardwick, issued a report based on the findings of inspectors accompanying detainees guarded by G4S staff on flights back to Jamaica and Nigeria. Hardwick said some security guards on the flights raised tensions by using force and restraint unnecessarily, while others used “highly offensive and sometime racist language” when talking to each other. (In response, G4S said it does not tolerate offensive and racist language among its staff, and that it had “received no complaints from the detainees on either of these flights”.)

And on it goes. It appears to be a shocking litany of negligence, abuse and indifference, reinforced by a lack of accountability. Mark Scott, the lawyer who represented Mubenga’s family at his inquest, says such tragedies are made more likely because the company uses zero-hour contracts. “Guards have to be seen to get the job done, to get the deportee on the plane, otherwise they are not employed the next day.” Although G4S subsequently lost the deportations contract, he says it had little impact on the working practices of guards who were simply transferred to a new employer.

So how can organisations such as G4S be made accountable for their failings? Employing security is always going to be a tricky issue – a certain kind of person is likely to be attracted to these jobs – which makes it all the more important that staff are thoroughly vetted and the company subsequently held to account. “There needs to be a mechanism for state institutions and the private companies they employ to be held to account when people die,” said Deborah Coles, co-director of campaigning group Inquest, after the Mubenga guards were cleared of manslaughter last week. “Neither G4S nor the Home Office were prosecuted for its failings to act on the well-documented concerns about the use of excessive force and racism.”

These tragedies have happened too many times in too many different situations in too many countries to not believe that G4S has a systemic problem. Is it systemically racist or systemically incompetent?

Yes, in extreme cases individual employees will be charged (and inevitably cleared) of manslaughter. But perhaps it is only when G4S knows that it will be charged with corporate manslaughter if somebody in their care dies unnecessarily, that they will ensure they recruit responsibly and learn how to look after people in their care without killing them.