Why is it that so much online discussion of Mangione's actions is supportive?
Wouldn't social desirability bias predict that people would be reluctant to support Mangione's actions in any way which might be traced back to them personally?
Hypothetical evil plutocrats would be far more likely to dig up your social media history to get your take on Mangione than somehow obtain your response to a telephone poll.
The supportive online climate suggests that supporting Mangione could even be more socially desirable. There is social pressure to support him.
In any case, your Harris/Trump example suggests that social desirability effects should be on the order of perhaps 2-3 percentage points in a poll. So it wouldn't represent a drastic change to the overall picture.
My theory: Mangione cleaves society along an "extremely online" vs "touch grass" axis.
The passionate social media addicts, who see conspiracies and exploitation everywhere, support Mangione.
The "touch grass" people understand that healthcare is a systemic problem which requires systemic solutions. Political murders undermine the social contract, increase the likelihood of subsequent political murders, and contribute to the unraveling of society.
The "touch grass" people are underrepresented in online arguments, but well-represented in a randomized poll.