There's what people say in private, and then there's what they say when others may hear/read what they said. So there's that (not too many people outside the closest circle ever hear any bad stuff that's said).
Other than that, I think people are most often decent and caring and don't want to say something disrespectful about the deceased person because of that person's family members/friends. When someone is "bad" (or even just a jerk to some people) that person not have not have been bad or a jerk to the people who think nice things should be said.
So I think most people tend to err on the side of being respectful to anyone who thinks the deceased person deserves some respect/dignity in death. It isn't going to kill anyone to either say nothing or come up with something respectful, at least in some circumstances and for a certain amount of time. There's a different time and place for bad-mouthing the person or saying negative stuff. Indulging in some need or wish to "also highlight the bad stuff" about someone who dies serves no purpose most of the time. In some instances maybe it does, but I don't think that's true in most instances.
I think most of the time people either stay away/stay silent completely or else say something positive (or positive-ish) because it's the decent thing to do while family/friends mourn. Besides, much of the time most of the bad stuff that really isn't a secret has already been talked about (so a) why re-hash it, and b) doing that serves little purpose).