Doctors and nurses have to take an oath on registration to "first do no harm". I would take this to mean that if they have the skills to help someone in danger they should do so.
The UK nurses I know don't, and as far as I know haven't, had to abide by the Hippocratic Oath. Just medical staff, as far as I'm aware. Is this different in other countries? Curious now.
I think the question of denying care based on lifestyle choices is indeed a slippery slope, but it's already happening sadly. A friend who is of big build, a familial trait, is now classified as clinically obese because she's shrunk with age. Walking is painful because of osteoarthritis, after years of physical work as a nurse and midwife, but she's been told that she will probably not be accepted for hip replacement. She's a nonsmoker, minimal drinker who eats sensibly The rules are already affecting many clinicians' decisions.
I understand where the OP is coming from, I am also furious with the flat earth, anti-everything types but as for stopping their treatment this won't happen. However, unfortunately, for more non-Covid related conditions I'm afraid the stable door has already swung wide and the horse is long gone.