Riddle sought a scape goat for the attacks he himself caused. His plan would have been to present Aragog as the culprit and then promptly stop the attacks. I think that having the the attacks stop immediately after a deadly beast was discovered and disposed off, would have been too much of a coincidence for the administration of Hogwarts to ignore. This is compounded by the fact the Riddle knew himself to be virtually (with the exception of Dumbledore) beyond reproach in the eyes of Dippet. Which would make it un likelyunlikely that he would not suspect Riddle of any foul play.
Furthermore , Hagrid is about 60 years old at the time of the second book (1992) which makes his altercation with Riddle take place at around 1945.
The first edition of Fantastic beast and where to find them is released at 1927, just 18 years before.
As a rule the Wizarding world is quite conservative, so I'd venture the book hasn't proliferated extensively in that time.
It could be argued then, that the methods by which Basilisks and Acrumantuli differ at killing were not widely (if at all) known.