lullabyknell:

On the subject, I want to share my personal favorite “Tom Riddle is a massive idiot” move. Because oh, it makes me laugh. 

See, I do believe that Tom made an earnest effort at protecting the diadem based on his knowledge. It’s just that his knowledge is wrong (and also that he was rushed for time and possibly that the Room of Requirement swallowed whatever protective spells he cast on the diadem). 

I don’t think that Tom was arrogant enough to believe that he was the only one who’d found the Room of Requirement, but I do believe that Tom is arrogant enough to believe that the Room of Requirement would somehow ultimately be loyal to him and him alone. The Chamber of Secrets and that no one had discovered his horcruxes primed him to believe that Hogwarts (his birthright, in his mind) would serve his interests above all others and that the diadem would be lost in this labyrinth of hidden things forever. 

But, see, not only does Tom not own Hogwarts, but Tom hid the diadem in a room designed to give up whatever any person desires. 

Sure, there might have been protections, but Harry Potter walks in and goes, “I want a horcrux.” And the Room of Requirement basically goes, “I have a horcrux!!!” and practically throws it at Harry’s head with no charge.