There are only three scenarios worth investigating
Scenario 1: David Bain murdered his entire family in the early hours of the morning, went out to do his paper round, then returned, turned on the computer, wrote the famous message "Sorry you are the only one who deserved to stay"
Scenario 2: Robin Bain waited for David to leave on this paper round, murdered his entire family, turned on the computer, wrote the message and then committed suicide, and David arrived home to find them all dead
Scenario 3: Robin Bain waited for David to leave on this paper round, murdered his entire family. He then turned on the computer, and wrote the message just as David arrived home to find them all dead. David and Robyn fought, a David killed him.
There are two possible times that the Bain computer could have been turned on...
Martin Cox, a software engineer from the University of Otago worked out a start time of 6.41.06am, however his methodology involved some assumptions and was hampered by the fact that the computer clock was correctly because the BIOS battery was flat so the clock only started when it was turned on. He also had to use a Policeman's analogue wrist watch that was two minutes fast and had no second had as a time reference to work back to when the computer might have been started. It is difficult to see how Mr Cox could come to such an accurate time given the inaccuracies in his measuring equipment.
Another computer expert Maatern Kleintjies employed more sophisticated technology than Cox used, said that the time could only be established as somewhere between 6.39.49am and 6.49.11.
If we take Joe Karam's assertion that whoever turned on the Bain's computer is the murderer, then we are still no wiser because David's estimate was that he arrived home at around 6:42/6:43, and this was confirmed by a neighbour, Denise Laney, who saw David at his gate just before 6.45 a.m.
If we take Maarten Kleintjes estimate as accurate, then the computer might just as easily have been already on when David arrived home, or it might not yet have been turned on. This would fit any of the three scenarios
If we take Martin Cox' less than satisfactory evidence, the computer switch-on time could only have been before David arrived home. This would rule out scenario 1
Essentially, the estimated computer start times are meaningless, and the two murderers theory is not ruled out by any of the possible start times