For me, it was that Roy Thomas Savage Land story in ASM #103-104. JJJ a field man? Spider-Man's greatest foe is quicksand? Gwen Stacy being brought along just for cheesecake? Kraven choosing Gwen for his queen? Peter shooting a gun? Killing Gog? Kraven not believing in honor?
What a crock! Spider-Man has been dead to me since that time.