It wasn't, we have a Ripper-esque maniac committing murders in service of a Master, the MO is organ removal and possibly a nautical motif. Fel refers to a Master, we don't know if he's working alone, but the letter implies the Master is doing the murders, with Fel facilitating them.
The Master is refered to as odious, a 'noxious angel', a 'vulgar fiend' and the valediction invokes pesitilence. So if we're looking for a monster, demigod, or avatar of a god doing the murders we have a few clues.
If the Ripper parallel holds, then Fel is the theoretical second person, the possible coachmen who is thought to have written one or two of the letters sent to the police.
The Crushing Wave worship Olhydra, who is referred to as a princess and doesn't seem to have any pestilence associations. Sebek unless we've got a Discworld Noir situation probably would show up in BG, the others also don't really line up
