Joe Karam argues that whoever turned on the computer must have been the murderer. The evidence does show that this was most likely turned on before David returned home from the paper run, which means that David is not likely to have been able to turn it on, and that Robin must have. Karam also argues that the computer belonged to Robin and he was very computer literate, introducing computers into his schools. There is no reason why the person who turned should also be the person who committed the murders.
As a person who earns his living by working at home by remote from a computer, I, like Robin, also turn on my computer first thing in the morning. However, I do not sit down in front of the computer and wait for it to boot up. No, I have better things to do with my time. So, I usually go away from the computer and prepare my breakfast. Experts examined the computer and determined that it would take over a minute to boot up and a further minute for MSWord to load and enable Robin to begin work. It is quite possible that, like me. Robin was in the habit of doing something else while the computer booted up.
Evidence shows that Robin did not go to the toilet and there is no evidence he went downstairs, however someone did bring in the newspaper and David Bain testified that he couldn't remember if he did or not. It is highly possible that Robin turned on the computer and then went out to collect the newspaper. During that time, David could have arrived, and passed him on his way back, then snuck into the computer alcove to ambush his father.
The computer turn on time is a red herring. Whoever turned on the computer is not necessarily the murderer in the Bain murders.