Why is hating humanity acceptable?

A good starting point to answer this would be to ask another question, "Is misanthropy more common today than in the past?"

I suspect three factors play a big role:

1. Lack of historical weight—Genocide and ethnic hatred only became acknowledged as the evils they are after the horrors of the 20th century. Run-of-the-mill misanthropy has rarely been the driving force behind large-scale atrocities. This makes the taboo it holds much weaker.
2. Doomer mindset—The average person today, particularly those in young adulthood, seems to have a more negative outlook on society, progress, and life in general. Frustration over climate change and other manmade disasters may also contribute. Misanthropy can easily manifest from these feelings.
3. Isolation—Speaking of society, misanthropy is probably closely correlated with a sense of disconnectedness from society. With civic engagement dropping and loneliness rising, misanthropy may be an outlet to vent frustration.