Oh come on - why seek to hurt/destroy a viable business in these dark days just because you personally don't like how you're treated?

Determine a fair value for your work and negotiate. It's a shame they opened with a low-ball offer backed by a threat (if this is truly how this all unfolded), but be the bigger man here and forge some cooperation! Maybe then they can learn a lesson about dealing with API devs without first seeing them scared off by a personal vendetta campaign