Oh, you know how I hate the idea of Snape being evil, and I have to confess that
this essay, by John Granger, scares me.
On the other hand, if I read an encyclopedia text on Septimius Severus, like
this one, I don't find many similarities with our Severus besides the annoying coincidence (?) of the names "Albinus", "Niger" and "Severus".