This “Angel of Mercy” usually kills victims because he / she decided they should no longer suffer from their illnesses. Also known as the medical killer, this type of murderer usually works in a hospital or doctor’s office, concealing their kills under the guise of natural causes.
The BAU caught one—Dr. Stanley Howard—who was killing his psychiatric patients with their own crippling phobias. The team was called in after he drowned a young man terrified of water, but was quick enough to save a claustrophobic woman from being buried alive.
"Scared to Death" (Episode 3, Season 3 of Criminal Minds)