Was he dark or just trying to be responsible and keep his head out of the sand in the face of massive challenges?

He wrote an essay about people being euthanized after age 63 in order to relieve the environmental strain of the high population. I don't know if he really believed that, but if he did and saw his health and quality of life deteriorating rapidly, then it is possible that he literally was trying to serve as a role model to people of how to be a good citizen and fight climate change.

I personally hope that we don't have to resort to such things as a society. But I believe that resource constraints and climate or other challenges are much more severe than people understand. I hope that we will be able to leverage technology to avoid disaster.

Intelligent people are able to understand and solve problems. That's why they don't ignore them and hope they will go away, like many less intelligent people. Brain might have been demonstrating a "last-resort" but effective solution to these types of global challenges.