I always heard this title attributed to Josef Mengele, and I know, take everything on Wikipedia with a grain of salt, but I found a video about America's serial killers and a woman named Jane Toppan, who poisoned 31 patients between 1895 and 1901, who was quoted as saying she wanted to have killed more people, helpless people, than any man or woman who ever lived, also had this title hung up over her name as well.

Anyway, I typed in Angel of Death to see if it had any one definite result, no, several: Mengele of course, also Donald Harvey, an orderly who claims to have killed 87 people though the real count is estimated between 37 and 57, including his neighbors instead of just patients

Colin Norris, a male nurse who killed 4 elderly patients in 2002

Kristen Gilbert, a nurse who killed 4 people at the Veterans Affairs Medical Center where she worked in 1996

Beverly Allitt, a UK nurse who killed 4 children and attacked 9 others in 1991 and got 13 life sentences,

and two other German SS officers: August Miete and Irma Grese.

This came as a surprise to me since I thought only one person was ever actually referred to by this title.

no.. the contemporary usage is pretty much a caregiver or medical professional who kills people who they are caring for (usually but not always terminally ill). The "angel" comes from their supposed good intentions---as twisted their motives may be. Mengele was in fact a doctor, so perhaps that "angel" reference was applied for that reason--regardless of the fact that he was a nut and a butcher.

Some of them also have the hero syndrome... where they purposely kill the patient so that they can revive them and be the "hero". I'm pretty sure that reference was made in a L&O epi that you would have seen.