A surgeon and an assassin both wield a blade, and both occasionally kill. Their intent and motives differ strongly. Mens rea exists because reasons. (This is

A surgeon and an assassin both wield a blade, and both occasionally kill. Their intent and motives differ strongly. Mens rea exists because reasons. (This is also why surgeons generally request a statement of consent to medical procedures from the patient, where that's tractable, or by a representative --- parent, guardian, someone granted power of attorney, etc --- where the patient themselves is incapable of consent. There are still some loose edges, but this itself is a significant though not sole distinction between the two roles noted.)