It is a crucial point to the credibility of omnivore diets vs. vegetarian to note that disobedience was the norm — in fact meat may have been viewed not just as something nice but as an overtly pagan act.
I would agree on the last point you make, it could very well have been regarded as a pagan act, act of rebellion, etc. But again, we don’t know as there are no affirming statements of it being done before the flood, and with those people being evil and not possessing complete knowledge and separation from God, they could have also just as easily come to believe eating meat was a dangerous thing to do, as inevitably if it was tried people may have died eating spoiled meat. Maybe they thought the other animals and insects may be okay doing it, but not people. You have to realize since God didn’t permit man to eat meat yet, He’d have shared no knowledge of preparing meat, storing it, etc. On the other hand we see that when He instruted man to make burnt offerings to Him He did spend time explaining how to make a proper offering, on what, what to use for a proper offering, etc.