The stranger in "The Masque of Red Death" was The Mysterious Stranger.
Both were mysterious figures that wear masks but are non-corporeal beings. Also, both have no problem killing humans for fun. The stranger in "The Masque of Red Death" was yet another Mysterious Stranger.
The Red Death was inspired by a viral hemorrhagic fever similar to Ebola or Marburg.
Ebola, Marburg, and related filoviruses would have been endemic in central and west Africa going back many centuries, and could have been familiar to the slave trade; although the legal slave trade had ended by the time the story was written, it was still going on under the table, and there were plenty of people who had engaged in the trade who were still alive who could have told stories.
