Am I meant to work alongside a person who has literally said he wants me to die, completely seriously?
So at some point, there is a line - I feel the more interesting discussion is why you feel this isn't beyond the line - presumably it comes down to a matter you not believing there was harm caused?
For me, I think the obvious justification for the firing here is that this guy had hiring as a part of his responsibility, and this document is a statement he isn't going to be able to fulfil that.
If I'm a developer and I write a manifesto about how we should be writing code to optimise for the worst maintainability to keep job security, am I fair game to fire? If I'm literally coming out with a manifesto that says I'm not going to do the job I was hired to do (of part thereof), is it unreasonable to fire me?