I have seen this a lot lately in online discussions of homelessness and people accused of crime. It's very unsettling.
That is the context of why Mr Tan wrote that -- there's a popular narrative that specific individuals are complicit in crime and homelessness in San Francisco. This leads to lots of ad hominem and in my view rises to the level of conspiracy theory in many -- it's not like every problem is the fault of a single office holder or even a "cabal" of them. Voting against someone or supporting different candidates is one thing. Calling them solely responsible for all that you consider evil, escalating to the level of death threats, is quite another.
And that's just politicians. It's also routine to see people call for violence on homeless people or people accused of crime.