"...A cat run over by a car."
Not entirely a bad analogy.
Ironic that you were exposed to him in high school, yet turned away. I've always believed that anyone who comes to his work at a later age won't become a fan for many of the reasons you state.
Why did I react differently?
Well, as I wrote before, he took the ideas, hints & suggestions of other writers & transformed them into a coherent universe with characters & rules. Much as Doyle did with the scientific detective, or Raymond Chandler with the hard-boiled detective.
Why that particular sub-genre appeals to me, I can't quite say, save that it does. The horror of scale, the unseen, & that which cannot be comprehended entirely. (Larry Niven's Ringworld, in another genre, appeals to me for many of the same reasons: the audacity of scale).
One very minor example: Recall "Imprisoned with the Pharaohs"? The protagonist, bound in rope, is lowered into the pyramid. His descent seems endless; his awareness fades. Finally he finds himself supine on some solid surface. As he shifts to free himself, he feels the rope at its source loosen, and coils fall on him. He continues to work to free himself (he is Houdini, after all). Moments pass. He makes progress. Then he realizes the rope is still falling, burying him. He can't imagine a rope of such length would even exist. He faints at the horror of it. When he awakens, the rope is gone.
Minor stuff. I know. And Houdini did offer the scenario, but that passage seems pure HPL to me.
Did he believe himself to be superior? Yes. As one of the last true scion of the first New England settlers. But I don't get a sense that he thought he was a great writer.
Some critics argue the awkwardness of his prose was intentional. A literary catachresis, if you will. Bending language in the way the walls of the Witch House bent in violation of basic Euclidean principles.
I wouldn't agree that he didn't live a meaningful life simply because he didn't get out much. According to biographer, L. Sprague de Camp, he wrote some 100,000 letters in his life, mainly to other writers. There are worse ways to interact with the world at large. I haven't read his collected letters, so I don't know if he was really an inept writer.
I used to consider HPL a personal taste. As my own copies crumbled on the shelves, I wasn't surprised to find his work, except for Arkham House editions, of course, largely out of print in the late 70s, early 80s. Now that has changed. Long after true hacks like Dan Brown, Robert James Waller & Dean Koontz are forgotten, Lovecraft with dominate 20th century horror fiction much as Poe dominates the 19th.
When I concern myself too much with an author's personal life I run the risk of it tainting their work. How am I to continue to enjoy, for example, "I should have been a pair of ragged claws scuttling across the floors of silent seas." if the author's anti-semitism remains in the forefront of my mind?
Ps: The image of you terrorizing your neighborhood with a trumpet version of Valkyries validates this entire conversation!