It is an indicator of guilt.
Pre massacre it helps to create his motives of hate and greed. The prosecution have to convince the jury he has a motive.
Post massacre it backs up his motive. As he spent freely straight away and showed no remorse.
This together with the forensic and circumstantial evidence gets the police a water tight case. No matter how much Bamber tries to break out.
Behaviour is NOT an indicator of guilt at all. It's only what YOU perceive as being guilt. Clever people give NO indication of their thoughts and never breathe a word to anyone. On the contrary their persona could well be that of being overly helpful,unnaturally pleasant as though a person has turned over a new leaf making it pleasing to others. That would have been his " acting " trait coming out and if that had been the way he was then I'd have said he was guilty. That's a schizo/split personality.
In a lot of cases,the murderer turns out to be someone who you least expected. This is why it takes skill in solving a crime such as this one at WHF.
There's nothing easier than pointing a finger to the person whose mannerisms,etc point to him being the culprit,but there's more to it than that and if EP had done their job properly,they'd have settled for the right person and not JB.