Why are so many creative people self-centered and self-destructive? Alcoholics and drug abusers. Self-pitying and self-loathing. Unfaithful spouses and indifferent parents. Is unrepentant narcissism part of the creative gene? Or is it learned behavior? Or...do you disagree with my premise? Are these the exceptions?
I don't claim to know enough to disagree with your premise necessarily, but I think one reason many writers seem monstrous off the page is that we hear about the monsters most often. Monsters are more newsworthy than law-abiding, upstanding people.
Also, it's possible that writers--who by definition get to act any way they want through their characters and fiction--are frustrated by the relative restrictions and monotony of life. For a writer, nothing gives the same high as writing. If they are unable to write for whatever reason, they try to replace that high with another.
I don't think this pattern of behavior is limited to writers. All of us would like to keep doing what we love most, and it's difficult to adjust to downtime or forced retirement. It's difficult, not impossible. I wish some of my favorite performers had made the adjustment.
While modesty forbids calling myself brilliant, I have my dark places as everyone does, and writers explore those places daily, not just for crime fiction or crime poetry, mind you, but for anything that involves conflict. By the same token, everyone has hopeful places. I write as much humorous stuff as I do dark stuff. As dark and crude as my poetry and fiction get, I won't resort to cynicism or insult here or in public. Despite disappointments, difficulties, and missed opportunities, I choose to be hopeful. Anyone who makes pessimism a life choice may as well check out now.