There're not a clear difference between both forms. At least not one that should worry you in the examples you propose, because it is true that the reflexive form with "se" means that there wasn't an external agent that provoked the death, so probably it's was due to a natural process. But the transitive one without "se" just aludes to the act while don't specify if there were an external agent or not. The truth is that you could hear both ones in a natural conversation in Spain. However, the following example would be weird: "Ella se murió asesinada por un ladrón". As the agent is clear in the sentence, the reflexive form is weird from a semantic point of view, but the grammar is correct thought. This would be ok: "Ella murió asesinada por un ladrón". Hope this will be useful!

"Morir" is another one of those verbs that can be used pronominal with a subtle, if any, change in meaning. Finding a satisfactory explanation as to why one version sounds better than the other is not an easy task. Grammar manuals tend to give vague reasons, to which there are no shortage of exceptions. The New Reference Grammar of Modern Spanish notes that "in formal written Spanish "morir" is used for all kinds of death. In colloquial and written Spanish "morirse" is less formal." As usual, the use of the "se" is far more frequent in spoken Spanish, while written Spanish tends to be more impersonal. Bear in mind the following points when using morirse instead of morir: 1. when the death comes about as a result of a violent or sudden external event, "morir" is heavily favored. 2. when reporting a death matter-of-factually, without emotional response, especially when focusing on when someone died, how many people died, or how they died, "morir" is preferred (far more common in formal/written Spanish). 3. In a figurative sense, "morirse" is the standard option. [See morirse de/por] 4. "morirse" is preferred when it is a natural death, especially when referring to the death of friends or relatives, where we are focusing on the subject of the verb, describing what the subject 'did', so to speak (far more common in spoken/informal Spanish). With the pronominal form, the verb "morir" tends to be foregrounded in the context and, therefore, is rarely used in subordinate/adjective clauses. 5. "morirse" is preferred when focusing on the dying process itself, seen as an entire event, rather than just "ceasing to live". For this reason, "morirse" tends to accept a progressive interpretation when used in the present simple or past imperfect, while "morir" doesn't. Hope I helped you Yiru. If you need a Spanish teacher visit my profile: https://www.italki.com/spanish

"Morir(se)" es un caso único en español. Cuando mencionas el sujeto, no hace falta el pronombre (se). - Ella murió hace dos semanas. - Se murió hace dos semanas. El (se) reemplazará el pronombre personal de sujeto en español.

Muchas gracias, Juan, Claudia, y Sergio! Sus consejos y ejemplos son muy prácticos para mí. Me ayudan entender el uso de las palabras.

Lo más correcto es usar la forma 'morir'. El uso del verbo reflexivo debe conservarse para muertes naturales o muerte de familiares y amigos.