Hi Philippe,
that's an interesting speculation! I seem to recall that back in the Sixties, Barnabas' look was often described as vaguely "Byronic," and I believe that Ruthven himself was modeled upon Lord Byron, with whom Dr. Polidori had a very complicated relationship (rather similar to Count Dracula having been modeled upon Bram Stoker's sometime employer, 1890s stage idol Henry Irving).
From very vague memories of reading The Vampyre back when I was in my teens (and you do realize we're going back to the Early Bronze Age here, so it was a loooonnngggg time ago), Ruthven is somewhat similar to the original 1967 characterization of Barnabas--unscrupulous, with a pronounced vicious streak, yet strangely attractive and undeniably erotic--perhaps it was in this sense that the critic you cite thought of Ruthven as "sympathetic."
My feeling is that there were variouis forerunners to Barnabas but the character brought these dispersed elements together in a new way. In the Universal 1940s thriller, House of Dracula, the Count arrives at the home of an Eric Lang-like scientist claiming that he is seeking a cure for his "affliction" (but I don't think that word is used). The medical explanation given for vampirism in the movie is similar to Dr. Hoffman's theory about a "destructive blood cell" which leads to disastrous consequences for the main mad-doctor protagonist in "House of Dracula."
The scenario of the vampire seeking a cure develops in a very different way in the 1940s film from how it played out on DS, but I do think of that movie as being influential in how Dr. Hoffman's storyline was plotted, and subsequent developments that grew out of that. In the first script in which "he" was supposed to appear, Hoffman is described as an austere white-haired scientist in his early sixties, similar to the scientist character in HoD.
G.