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Yes, such games are definitely harmful. Those who disagree, please look at the results of Bandura's experiments (he's a psychologist) with children regarding the influence of observed examples on aggressive behavior.
But not only these games are harmful. Television is harmful, books are harmful... damn it, all our literature and history are harmful! All games are harmful in which aggression is manifested in ANY WAY (of course, to varying degrees), including our beloved heroes!
Who doesn't know the jokes about the "doomer" who goes outside and gets under a car, being sure that he recently saved? Who has NEVER experienced a similar altered state of consciousness after playing for hours? So, Postal can easily induce such an altered state in which you'll slash everyone around you without thinking!
Regarding the period of senile dementia in the Great Russian population, I tend to agree with AlexeyPunk; and even more than that - I believe that a period of senile dementia is coming for the inhabitants of all economically developed countries. And this dementia is connected to the fact that the inhabitants of these countries are gradually becoming more and more detached from real life - there are no serious problems (threats to life from wildlife, threats of famine) for them, so people are starting to "go crazy from being too comfortable." Only God knows what this will end with... :(