You're right that it's not everything, but it's the bread and butter, whereas the stories about people coming together are niche products.
In their defense (and that of society at large), though, it's probably not a degeneration of humanity. AJ Raffles outsold Sherlock Holmes for a good while, don't forget. Short-term profits tend to favor sensationalism and grit, but by the same token, in the long term, they also get forgotten.
The forgetfulness may also play into the fact that everybody's always convinced that the upcoming generation is worse than the previous. We remember the da Vincis, not the dozens or hundreds of primary and secondary sources from the period of people entertaining themselves via animal cruelty (burning cats to death was popular) or public hangings. We remember beating the Nazis, not studying how to create firestorms on demand to wipe out entire cities. Long view, the bad can't maintain the focus.
I'm guessing the darker stuff is favored by writers and publishers more than anybody else. It's easy to sell sex, violence, political divisiveness, or just general unpleasantness and petty bickering. Those have always sold themselves, again, in the short term, so anybody can make a buck. But actually takes talent to write a sellable story about people helping each other.
It's starting to look like the Internet might be burning everybody out on the crap, though. In a world where nudity, self-absorption, violence, any ugly -ism you can imagine, and small-mindedness is all piped directly to your eyeballs 24/7 for free (because production costs have dropped in almost every market), who'd pay for more? So maybe we'll see a change soon and the anti-heroes will be a tiny niche like they should be for effect.
Or maybe not, and I'm grossly underestimating the market for people feeling vicariously transgressive...
