I don't really have a theory about this particular case, but I would caution people against reading too much into a suspect's attempt to keep evidence out of the public eye. We all think that we'd do anything we could to prove our innocence, but when you're actually in the hot seat things can look very different. Especially in a case like this where there's little hope of it being resolved to anyone's satisfaction, short of the killer making a credible confession.

I mean, let's assume for a moment* that it was a kidnapping gone wrong and the murderer was someone outside the family (say, a work friend who knew John's bonus amount). In this scenario, the parents really did have nothing to do with it. Do you really think that more of John's handwriting samples would deflect suspicion? Or do you think that amateurs who are invested in believing that John played a part would focus on vague similarities and believe even more strongly? I honestly don't think that even 2 totally different styles of writing could convince most people.

I'm not trying to call you out or anything, but I think we place way too much importance on the actions of the suspects and how we think we would act. The fact is, until you've been in that situation, you don't know how you'd act for sure. And a lot of smart actions are often read as suspicious. I have several lawyers and a lot of cops in my family, and I also assist in cadaver recoveries and missing persons searches through my work as a search dog handler, which means I know a lot of cops. Over the years I've become convinced that the best option is always lawyer up and usually release as little information publicly as possible (assisting in the investigation is a different story, of course) because, while media/public opinion isn't supposed to influence cases, it absolutely can.

And people do react very differently to tragedies, just in general. Although I do assist in criminal cases, the bulk of my work is pretty normal search and rescue--people who get lost or who suffer from a sudden accident/injury while hiking (heart attack, falls, etc.). I've seen their families at incident base and it's crazy how different people are. You'll have one father weeping like his adult child is definitely dead when we have no reason to expect anything other than a successful rescue, while another one is cracking jokes and making snacks for the rescuers despite the fact that he knows that we're expecting a cadaver recovery now. People deal with stress and fear and grief so differently that it's hard to make any judgments.

Sorry, that got a little rambly and off-topic but I just don't think you can read anything into the lack of publicly available handwriting samples. Maybe he's trying to hide something, but it's just as likely that he didn't write the note and doesn't want to fuel the speculations of a bunch of random people with varying degrees of expertise.