I'm not an expert in psychology, but within a fixed population window, the murder rate will only go up unless there is a limited supply of those who murderers want to murder or unless murderers tend to prefer murdering other murderers, perhaps due to the change in game theory creating a new incentive. As humans would begin to live ever increasingly far away, we may approach a "murder death" condition where no new murders are possible. I will leave alone those who played Among Us until their numbers diminish and there is a better chance of reaching an early asymptote.