Fighting in prisons has been unofficially but actively sanctioned because it made life easier for those in charge.
Occasionally, it transpires, a guard will get in and have a go himself - or, at least, lend his advice on technique.
What clearer evidence could there be that violence behind prison walls has the approval of those running the place?
Prison is where society sends its mistakes. Everyone inside is there because society has failed them.
The case of the 13-year-old boy who endured the most squalid of upbringings before stabbing dairy owner Arun Kumar is extreme but not unique.
He is just one of hundreds of people who grow up knowing nothing but violence and cruelty where there should be love and nurturing. Plenty more such children live on our streets, forging lives of desperation, their ultimate incarceration only a matter of time.
Another considerable proportion of the prison population is made up of people who are there because we no longer have hospitals to cater for the mentally ill. Those who can't find a place in the community are locked up where they won't be a bother - to us, at least.
That number also includes those who committed crimes under the influence of drugs, including alcohol.
Drug addiction is a mental illness that should be treated as such.
A substantial amount of prison violence is gang-related, but gang membership itself is the result of social failings.
And even so-called white-collar criminals - those, for instance, who couldn't stop helping themselves to other people's money - should be seen as the product of a society that values prestige and possessions ahead of trust and honesty.
And although the question of private or public ownership of prisons is tangential - the violence and the way it benefits the system were a reality long before Serco added Mt Eden to its investment mix - it matters.
Personal freedom is such a fundamental right - without which few other rights can be protected - that to deprive a person of it is something for which society as a whole should take responsibility.
But a prison is not a pound for humans, where we abandon animals we can't control any more. We are all responsible for them being there as long as we support the system that puts them there.
So we should all be responsible for what happens while they are there.
We shouldn't blame Serco for what happened at Mt Eden. We should blame ourselves.
While we're at it, could we go back to calling Mt Eden a jail instead of a Correctional Facility, a hideous Orwellian euphemism that makes it sound like an academy devoted to maintaining standards of spelling and grammar, instead of what it really is - a disgrace that shames us all.
