For that matter, Nicodemus has been around longer than Tessa, and he's tried to go the plague route at least once in modern times. Who knows how many he was involved in.
And can we discount any of the Vamps? I mean, the Red King has to have a massively impressive kill-count. Think of all the sacrifices over the years, and he suffers from an abnormal bloodlust as well? That guy has to be up there in individual/commanded kills.
Individual kills is probably Ebeneezer, but we don't know how long Kincaid has been around, do we? If he's been around a couple hundred years, he's going to have some impressive numbers. Maybe not on Eb's level, thanks to some of Eb's bigger hits, but personal kills? Maybe near the top. Along with Mavra.