I think it's a reasonable guess that the majority of serious abuse is a small number of repeat offenders simply because that's how it is everywhere else. Most criminal activity is the same way. It's not like every person steals one car or commits a burglary in his lifetime. It's a small percentage of people who do it over and over again who run up the stats.
Yes and no. Yes, it's likely a minority of the population who account for a large percentage of "actual crime," and I quote that to discount the nonsensical crimes of possession upon which our political heroes have spent decades wasting incalculable resources. No, in that the inferred parallel of cops to the general population isn't a valid one. A much more apt comparison would be to liken police to an organized crime outfit or terrorist operation, as the very institutions themselves are engaged in criminal pursuits of varying degrees of severity. Within that paradigm, most or all of the involved individuals are guilty of crimes, unlike within the general citizenry.
The problem that seems to be more universal is the willingness of all of the other police to cover for the worst offenders. A cop who probably wouldn't unnecessarily beat a suspect still seems very likely to lie to protect a fellow officer who would.
The Mafia use the term omerta within their ranks to describe this code of silence, reinforcing again the parallel between police and organized crime syndicates.
Weirdly, police spokesmen like to use the phrase, "A few bad apples..." to describe the problem. They don't seem to know what the rest of that saying is or how well it applies to them.
Largely because the audience to whom they're speaking are either ignorant or uncaring of the rest of the axiom. Believe you me, the bunch has been spoiled for a very long time.