In the US, doctors are obligated to treat patients in immediate need of care (in a professional setting - an emergency department, for example - not just walking down a street.) They can’t discriminate against patients for non-clinically relevant reasons (race, gender identity, etc.) They CAN refuse care if they lack specific skills or the patient is “abusive.”

HOWEVER, these are ethical obligations (I pulled that info from the American Medical Association’s Code of Medical Ethics.)

You asked about legal obligations.

I am not well versed in doctors’ legal duty of care - laws are not consistent across national and local jurisdictions.

You also used the word “aid” so I am approaching it from an emergency context.

In a professional setting, there are limited reasons a medical professional could refuse emergency care where the immediate outcome is death. Perhaps someone with more legal expertise could direct you - I’m only familiar with ethical constraints.