The final result of a medicolegal death investigations is the decision regarding the cause and manner of death. The Medical Examiner or Coroner do not rule a death a murder, they will however rule some deaths homicides. Murder is a legal-criminal charge, it is for the lawyer-types to make that decision.
Homicide, simply put, is a death of an individual at the hand of another. To make that ruling the investigation demonstrates or decides that that death was the result of a volitional act, the individual decided to do the action. The individual who causes the death must also be aware that their actions would likely result in another individual’s death. Nonetheless, a death at the hand of another can also be accidental. While most often the distinction is obvious it can be a bit tricky at times. It is clear cut when one individual takes a handgun points it at another person and pulls the trigger.
As an example, consider a drunk, or otherwise impaired or distracted, driver who causes an automobile crash. Most of the time that would be considered accidental at its core. The individual didn’t consciously think that “if I drive under these conditions I am going to kill someone”, although they should have at least considered the possibility that they could crash their vehicle. Killing someone would likely not enter into their thoughts, certainly they didn’t think “I am setting out to kill someone by drinking heavily and getting behind the wheel of my car”. On the other hand, consider an individual who consumes large amounts of alcohol knowing he is minutes away from driving a bus full of folks on a cross-country trip, well aware that he is impairing his driving ability. When that individual gets in a crash of his making it could very easily be determined to be a homicide. The ME or Coroner could rule the deaths in that crash as homicides even if the driver was not charged with a murder by other authorities. The death at the hand of another is there, as is the volition/decision by the driver and the knowledge that his actions could/would result in the deaths of others. Gross negligence could raise the death to the level of homicide for the ME, but not to the level where prosecutors move forward with a murder charge.
Consider a case I read about some years ago. A woman came to a hospital ER complaining of symptoms consistent with a heart attack. She looked to others in the waiting room to be very ill, but the triage nurse, recalling that the individual was an illicit drug user, tells her to wait in the waiting room. She suffers a cardiac arrest within minutes, right there in the waiting room. Was that a homicide? Did someone decide not to render needed care knowing that that lack of care could result in death?