The message left on the computer was a controversial piece of evidence that was one of the items cited by the Privy Council for a retrial.  In this case there was conflict over what time it was switched on.  Regardless of timing the message read:

"Sorry you are the only one who deserved to stay"

If you believe that David killed his family and wrote this message then it is understandable that he didn't want the message to be traced back to him, which would happen if he handwrote it, and he wanted to create a scene which made it look like his father was the culprit.

If you believe that Robin killed his family then wrote this message then there are one or two things to take into consideration:

1. If you are going to kill yourself why bother to write a computer message, why not just a freehand note?
2. If you want to exonerate your eldest son,  David, why write a computer message which might lead him to be implicated for murder when a handwritten note would completely exonerate him and show that you in fact killed the family and yourself?
3. Why would you consider your eldest son more deserving when you have a daughter who was dux at Bayfeild high school in Dunedin and doing well at teachers college, and another son who is showing a lot of promise?
4. If you are going to leave a suicide message it is more likely to be a message about yourself and your motivations for what you are doing, rather than about someone else.  The content of the message itself reflects the mind of a pathological killer who has made the decision that he is the only one that deserves to stay, a course of reasoning which provides him with the justification he needs for the act which he has perpetrated.