LONDON — A neonatal nurse charged with murdering seven babies and attempting to kill 10 others was accused in court of injecting newborns with air and feeding them insulin at a hospital in the United Kingdom. Prosecutors accused Lucy Letby, 32, of being a “constant malevolent presence” at the hospital in northwest England in a years-long case that has sparked horror and fascination in the country. She has pleaded not guilty. Prosecutor Nick Johnson told jurors that Countess of Chester Hospital, where Letby worked, saw a significant rise in deaths and “catastrophic collapses” in its neonatal unit during 2015 and...