That's not my experience - at least where the sort of really excessive violence of this case is concerned (I agree that they do tend to resort to some violence pretty quickly during robberies, etc. but not, in my experience to this sort of thing)
As I said from the start, I have only experienced this as a matter of identifiable trend in relation to sexual motivation (often linked with an aspect of hate crime, now I think of it) and mental illness.
I don't know all the details of exactly how they were mutilated so you could be right
There are some kinds of injuries that obviously suggest a sexual motivation and some that indicate mental illness. The postmortems stabbings would suggest either possibility as they serve no function in terms of torturing someone to obtain information and there is substantial literature recording such murders as being motivated by sexual sadism and/ or hate crime.
However the fact that the murderer/s set fire to the crime scene suggests that he/they had attempted to destroy evidence yes? It is possible that the postmortem stabbings were an attempt to throw investigators off the case, yes? Especially given that, with the CSI effect, an ordinary member of the public can learn about how to disguise an acquisitive crime as a crime committed by a sexual psychopath.
The fact that a laptop had been stolen at an earlier burglary suggests that,
if the murders were linked to the earlier burglary, the perpetrator/s had some time to think about what they were going to do and how to cover their tracks, so there is a
possibility that he/they planned to make an acquisitive crime appear to be a sex crime or some other kind of crime.
Another possibility is that more than one perpetrator was involved as there is a history of horrific murders, involving kidnap and torture where the relevant factors were a) a group who decided, for whatever reason, that they were justified in torturing people to death (or at least in torturing them, got carried away and then realised they had gone too far and went on to murder) and b) the consumption of certain drugs by the perpetrators
The cases that come to my mind to me are the murders of Mary-Ann Leneghan and, years earlier, the murder of Suzanne Capper. Both murders involved horrific torture and sexual degradation of the victims while the perpetrators were under the influence of drugs (crack cocaine and amphetamines respectively). In both cases the perpetrators were fired up by a sense of righteous indignation in relation to real or imaginary crimes that they imagined the victims had committed. In both cases the members of the group egged each other on to commit even more horrific acts of violence that, as individuals, they probably wouldn't have committed.
Just thinking aloud really. I really hope that the man who gave himself up is the perpetrator.