Edit: Here’s a link to what is most likely the real manifesto: https://www.kenklippenstein.com/p/luigis-manifesto
Ken Klippenstein is a very reliable journalist and this version of the manifesto contains the snippets that have been released by law enforcement. Also, considering the thing was hand-written, that very long version involving his mom is dubious. (And there’s not any good evidence that his mom is in anything besides decent/good health)
Socialized medicine would benefit every single sector of the economy besides the insurance industry. It would deradicalize millions of Americans in an instant. It would increase the amount of doctors and nurses and healthcare workers tremendously. It would save them tons of money. But the government won't do it because they aren't powerful enough to tell roughly 20 of the 1000 most powerful executives in the country to fuck off.
Unfortunately, a lot of doctors oppose it as well because the increase in the number of doctors would dilute their negotiating power.
I have only ever heard opposition from doctors (or medical students) that were right wing/conservative to begin with. Most of the doctors I've worked with (especially those in emergency medicine and family medicine) are very much in favor of universal healthcare.