There are at least 4 situations in which the failure to act may constitute breach of a legal duty. One can be held criminally liable: (1) where a statute imposes a duty, (2) where one stands in a certain status relationship to another (Parent/child, clergyman/clergy member, doctor/patient), (3) where one has assumed A contractual duty to care for another, and (4), where one has voluntarily assumed the care of another and so secluded the helpless person as to prevent others from rendering aid. There is maybe a number (5) when a person creates a risk of harm to another.
Remember the Doctor life support case: the court determined cessation of heroic life support measures is not an affirmative act but a withdrawal or omission of further treatment. meaning, there had to have been some legal duty to continue the life support. There is a duty if the medical treatment is proportionate (benefits outweigh the burdens to the patient, even if it causes pain if the prognosis is barely above 0) Here, there was very little chance to improve the condition of Herbert. Medical professionals said so