Secreting Helen and Boris in the cesspit within the family home was probably IS' biggest mistake because it proved she had been murdered, and most probably by him. I guess the degradation of Helen's body in human excrement may be more a matter for the judge, as he could regard it as an aggravating feature when it comes to sentencing.
I believe in the context of the crime as a whole (and its background), the cesspit reveals a great deal about IS. It takes a special kind of ruthlessness to kill your fiance, the person who you were discussing wedding venues with just hours earlier, then drag her corpse to the most revolting location imaginable, with some effort force her body inside there, (and that of her dog), and then shut down the lid. Later that evening IS ate a Chinese takeaway and chatted with his son, just yards from that filthy tomb. This is the very opposite of a crime of passion - it is dispassionate and cold blooded in the extreme.
The cesspit exposes his dishonesty and deviousness - it proves not even the conventions of basic human decency would prevent him from covering his tracks. Once the jury has agreed no one but IS could have placed Helen's body there, his lifelong charade of being a decent man crumbles into a million pieces. When he was on the stand, he himself described Helen's murder and disposal as 'Sick and disgusting'. Those words sum him up, in my view the cesspit clearly demonstrates what a sick, disgusting and dangerous killer he is.
