Harters I agree his attitude in Court was not normal for a man on trial for murdering his entire family it should have been a time of sadness not being cocky and arrogant. I think he was putting on a "show" for the newspaper deal he thought he was going to get and he did show himself in a very bad light.
Julie's comments are veyr illustrative in thig regard.
She said that at one point she threatened to go to police and he said go ahead it would do nothing because he was what we would call today "bulletproof".
He seemed to honestly believe they would not be able to make a case against him so he saw no reason not to act like an arrogant jerk and even to toy with police.
He grossly overestimated the competence of his frame job and let his natural personality come out.
Had it not been for the evidence that he didn't think of the arrogance would not have been enough to convict him. So he saw no harm in it. In a way he was right.
That arrogance did not lead to evidence that ended up convicting him (save being seen entering through windows which he admitted to anyway) so it merely serves to further illustrate his personality to people interested in the case.