The motive according to the prosecution seems almost insane, but given how much AM had got away with, and for how long, before that time, I can see the reasoning- if he can just buy some time..
To me, considering the incredible stress he was under- his father dying, his mother suffering from dementia, the boat crash trial, and being confronted at work that day about his thefts- it seems unbelievable that his account is that is was pretty much an ordinary night-driving around with Paul, napping in front of the tele, visiting his mother. The gig was up at work, it was the end of life as he knew it, and he didn't react to this? That narrative seems almost as incredible to me as that of a loving husband and father murdering his family.