Philo Vance, Ogden Nash famously said, deserves a kick in the pance. That motion enjoyed large support over the years and it's easy to see why. An arrogant, pompous, self-centered snob with an oyster's sensibility and a quite personal sense of ethics, not to mention a seemingly endless erudition, there is no question Vance is an unpleasant fellow... just like most of his predecessors and contemporaries. Van Dine's work was essentially derivative and his sleuth is no exception. All of Philo's "negative" features were borrowed and aggrandized from more or less famous models; they were basic requirements for great detectives back then.
Modern readers expect their favorite sleuth to be a regular, fallible human beings whom they can "relate" with: Alan Banks or Kurt Wallander may not be the smartest investigators in the room, but their personalities and experiences are close enough to the reader's to allow him* to identify and bound with them. Their love affairs, health and/or family problems end being more crucial to the books than the cases they investigate, competently but not brilliantly. Such an idea would have left an Anna Katherine Green or a Conan Doyle scratching their heads in bewilderment. Detectives back in the formative years of the genre were rarely regular, virtually infallible except for some exceptions that confirmed the rule and the only feelings they were supposed to elicit were amazement and admiration, the "Watson" serving as a cheerleader. Fans worldwide who lamented Sherlock's temporary demise mourned the great detective - they didn't mourn the man.
Because being likeable wasn't yet part of the job, early detectives as a rule were not. Only Sgt. Cuff can be called a wholly sympathetic figure; the rest is divided between coldly manipulative professionals a la Lecoq/Gryce and Dupin-influenced eccentric amateurs with a ballooned ego. Fascinating characters for sure, but also quite frightening at times; the face of Justice, in these times, was not a friendly one.
Not that they cared much for ethical issues anyway. We are often told that detective fiction is about restoring order, but it hasn't always been so. Belle-Epoque sleuths' motivations were remarkably mundane: money, getting oneself or a loved one out of trouble, or the plain thrill of the game; also, they weren't above letting the criminal go free if they sympathized with his motives. But at least they remained on the right side. Well, most of them. Think of Romney Pringle or, more sinisterly, Horace Dorrington. But the most chillingly effective portrayal of the dark side of the Great Detective figure is to be found in Baronness Orczy's stories featuring Bill Owen, better-known as The Old Man in the Corner.
Owen, if you don't know him, is generally regarded as the very first armchair detective. He does all of his sleuthing from his chair at a teashop where he meets his reluctant watson, female journalist Polly Burton, and the only physical activity he is ever seen indulging in is tying and untying knots on a rope or any piece of string he can find. Alike most amateur detectives of his time and beyond, he is basically a "thinking machine" for whom crime solving is first a kind of an intellectual game, a duel of wits (the Old Man stories were among the first, if not the first, to include challenges to the reader) and he takes much pride in his ability to see what nobody saw and guess what nobody guessed. But, unlike his colleagues, he doesn't serve justice - be it formal or a more personal kind. To unravel the murderer's scheme is enough for him and he doesn't feel any need to call the police (which he loathes) to tell them about his findings. Murder to him is a fine art and he thus sides with the masters of the craft, now matter how nasty they are. As to victims, who cares? The Old Man, in short, is a nihilist; one of the stories suggest he might even be a murderer himself - unrepentant of course.
It's easy to regard such a character as either a novelty act or an aberration. Easy, but mistaken. What Orczy shows us, voluntarily or not, is that the line separating the ubermensch from the downright inhuman is a very, very fine one; many Great Detectives might as well have been Master Criminals. The key-difference between Holmes and Moriarty or Wolfe from Zek is not one of personality. It's one of polarity.