It's possible to become a good guy after being a bad guy?
Of course. Why wouldn't it be possible?
He wasn't necessarily a bad guy. He did a lot of stupid stuff, but it raised awareness that we need to start thinking about the environment (nobody really cared back then). As he said, that goal's been accomplished. While the rest of his old comrades are still wallowing in the "awareness" mode (which is now just counterproductive self-aggrandizement), he long ago realized it's time to start working for practical solutions rather than just bitching about everything. We need more of him.