The tragedy is that they will get more government contracts. The saga of the last 20 years is the gradual dismantling of state services in favour of these giant, unaccountable companies (see also Serco). There isn't an alternative to the likes of G4S even if any prospective government wanted to not hand taxpayers money over hand-over-fist.
G4S is also at the heart of the case of Jimmy Mubenga an Angolan who was to be deported from the UK. Their staff forcibly held him down on board an airliner despite his pleas to be released and the complaints of other passengers. Mubenga died. G4S had previously been warned that its staff were using excessive force and were guilty of racism. Incredibly, the Crown Prosecution Service said there was insufficient evidence for a prosecution, despite several passengers on the plane being willing to testify.
Older UKers will remember, Group 4 as it was, repeatedly losing prisoners when they first got government work. Since then they've only continued to surprise us in their ability to find new and increasingly appalling ways of screwing up.
And don't think their mistakes are limited to the UK, G4S owns Wackenhut which has a less than eviable record running American prisons and protecting US military bases.