David Bain had a compelling motive to commit the murders. This motive is articulated by David himself in the booklet published by Joe Karam in 2001 called Innocent: Seven Critical Flaws in the Wrongful Conviction of David Bain. The following is a quote from David:
"I find this state of limbo depressing and often hard to live with [referring to being in prison], but it is eminently easier than being constantly crushed by shattered dreams, destroyed plans, broken promises and betrayals, by all I once held dear."
The first part of the sentence clearly refers to his imprisonment (this being written in 2001 when he was still incarcerated), however the second part clearly refers to when he was not in prison. It most likely refers to the environment he felt that he was in when he was with his family. We know that they were a close knit family and therefore "by all I once held dear" must refer to his family. Who else would this refer to? Tellingly, he says "once held dear" which infers that he no longer holds them with any affection. This is supported by observations of lack of emotion following the murders and the lack of any reports of David expressing any feelings of loss from having all his biological family killed or anger at his father for supposedly doing it. The complete passage in the Innocent booklet from which the quote is drawn is also absent of any reference to his family or any feelings of loss or anger in respect of what happened.
Further to the above, he says that his current "state of limbo" is "eminently easier than" the previous [family] environment, which infers that he does not regret the loss of his family. Being sent to prison merely frustrated his plans.
You might ask why David would write something that is so self-incriminating. One of the enduring characteristics of the Bain case is the amount of mystery in it, and that largely stems from the very idiosyncratic and eccentric nature of its key player. David came from a family of eccentric people, spent most of his early years being home schooled in a third world country with a mother who suckled her children well into childhood. David has given evidence that is often conflicting with itself and conflicting with the evidence generated by his chief supporter, Joe Karam. If David had been careful not to say things that incriminate himself, then he wouldn't have said that he was the only one who knew where the spare trigger lock key was and he would have made his testimony conform to that supplied by Mr Karam in respect of things like the alleged incest and his father being separated from the family. We just have to accept that David is who he is and says what he says. He may be intelligent but he is also odd.
"I find this state of limbo depressing and often hard to live with, but it is eminently easier than being constantly crushed by shattered dreams, destroyed plans, broken promises and betrayals, by all I once held dear."
David's whole life revolved around the family and he was, at age 23, still living at home. In his interview with Justice Binnie he explains that it was because the family was so close knit. But we also know there were major tensions within the family, including between David and his father. What are the "shattered dreams, destroyed plans, broken promises and betrayals" referred to here? We know that David and his mother were involved in plans for the new house and David had spent a year or more at home, doing little more than dwelling on those plans with his mother and spending a lot of time on the section clearing trees and creating a garden in preparation for the new house. There was testimony in the retrial that David saw himself occupying the master bedroom opposite his mother in those plans. This indicates a strong bond with his mother and a desire to supplant his father as head of household.
There are, however, indications that David had an argument with both of his parents about him moving out of the family home to make his own way in the world. He related to his then girlfriend that moving out of home was "out of the question". In addition, there are indications that his mother, Margaret, was shifting her attentions increasingly away from David and that she was admonishing him for his treatment of his siblings. One can only speculate why Margaret changed her focus, maybe she realised that she needed Robin's help more than David's to get the house built and so decided to side with him instead. The result for David was betrayal. Betrayal of all that work he had done and all those dreams, plans and promises.
David had another reason for resentment and that had to do with the performance of his siblings, especially Arawa, who had recently been Head of School, and Stephen, who was now turning into a young adult and starting to attract parental attention. Meanwhile David had failed his first year at University in a course of study that his father foisted on him and had spent a year on the dole and had not much to show for himself.
David was controlling and manipulative and Laniet had already left home because of him. As his siblings grew older his ability to control them would diminish. When his parents were in conflict with each other there was a power vacuum in the family that he could fill. Once it became apparent that they were healing their differences and going ahead with plans to build the new house, then his position of control weakened.
A witness in the retrial testified that David had taken the fatherly role in the household, This immediately creates a situation of tension between him and Robin:
Q. Did Margaret say anything at all about David’s role in the family?
A. Yes she said that basically David had taken over being head of the house while Robin was working down at Taieri Mouth at the school. She said that he stayed down there for the whole week because it wasn’t worth his while coming home, it was too far, and that David had taken over the day-to-day, um, father things that the male of the household does.
The ultimate expression of control over other people is to kill them.
In his speech at Perth in 2012, David refers to his family members in the present tense:
"I can only thank my upbringing, my family, my Mum and Dad [who] helped us with our education, with our upbringing, with university studies and helped us become the people we are"
Having the ultimate control over his family now means that David can characterize them in whatever way he likes. Instead of holding his father in disdain for allegedly abusing his sister and then murdering his entire family, which one would expect from an innocent victim, David expresses nothing but admiration. There is no attempt by him to try and understand why his father would do something so unimaginably violent as to kill his entire family or no expressions of the myriad of emotions that follow from being victim to such an horrific crime.
By killing the family, David would then have full control of the house building plans and would be able to build it to his own specifications. The Bains at that stage owned the house and had other properties elsewhere. The current estimation of their worth at that time equates to something like $600,000 in today's money. In the retrial, a witness, who was a close friend of David's testified that David told her before the funerals took place, which was within a week of the murders, that David already had a plan for what to do with that money:
Q. Did you talk about the funerals in any other way as well?
A. Yep, he knew that he wanted Arawa to wear the ball gown that she was either going to wear or had worn at a ball. That he hadn't decided about the clothes for the rest of the family yet and that he was hoping to be able to get in the house the next day to be able to go and choose, but he hadn't particularly got any ideas about that. I’d asked about, you know, what his plans, you know, had he started thinking about what life might be like after funeral and he said that he was thinking he was going to sell the house and buy somewhere for himself so that he would have a home.
Q. Did he mention where the money was going to come from?
A. Yeah he was quite concerned that he managed to get the money back from the Meders, because he was a bit anxious that with the amount of money, if it, you know, kept growing with interest and what have you, they would not be in a position to be able to repay it.
Betrayal, jealousy, control and money. All the motive for murder.