I notice this keeps coming up.
No, hitting someone by accident and failing to render aid, will not lead to a murder charge anywhere that I know of. That is manslaughter: inadvertently causing death, no matter how carelessly or negligently.
Murder is not defined by statute, but by common law (tradition), so it is essentially the same throughout the English-speaking world, and the legalities were sorted out in courts hundreds of years ago in England.
The key to a murder charge is formally known as, in Latin, 'mens rea' - the guilty mind. Prosecution must prove the person knew what they were going to do was murder, before they did it. The murderer knew it was deeply wrong, but did it anyway, deliberately, with intent to murder. If the victim survived, it was by accident, because they meant to kill them dead.
Another way it has been described is 'with malice aforethought'
No 'whoops, I got distracted, I didn't mean to stab him in the heart', 'oops, the gun went off as I vigourously cleaned it, it just happened to be aimed directly at her head from 2 feet away,' etc.
en.wikipedia.org
It's really not that complicated. I don't understand how, before the arrest, everyone was quite happy to speculate that her husband was a murderer, but seem so sqeamish about it with regards to the real suspect.
JMO