It is not normal and should not be normal for major political donors to make public death threats to local officials in the city where they live, even as a joke. It's toxic and corrosive to society and politics, and makes him seem unhinged. Tan is rightly getting excoriated, and he deserves scorn from his own political allies for significantly damaging their common causes.
Tan keeps complaining about SF politics being frustrating, but in my opinion, as someone who supports a significant portion of his policy platform, local politics would be improved if Tan would just move somewhere else or shut up and keep his money to himself and leave political discussions to the grown ups.