Can you think of a story of classic literature where the mentor becomes the heroes greatest archenemy ?

That would depend on what you mean by "classic literature."

It's not uncommon in literature of the past couple centuries, but traditionally the term "classic literature" refers to that by the Greeks and Romans from the Classical Period.  A period that ended about 1800 years ago.

From more recent or popular works, I'd think of Saruman and Gandalf from Lord of the Rings (if you consider Gandalf *the* hero) or even Ra's al Ghul and Bruce Wayne from the Dark Knight films.  Certainly Fagin and Oliver from Oliver Twist.

Tolkein used this common trope. Sauron taught the smiths the art of ring-craft, but not how the rings included binding magic controlled by the One Ring, which he kept. He was also a full-on Evil Mentor to the last king of Númenor.

Not a classic, but George R.R. Martin made Littlefinger go from father-figure mentor to Sansa Stark to her archenemy, selling her to a man he knew would harm her irreparably.

If I were better read in the classics, I imagine I could come up with others, but as it is...