why does haydee refuses to leave the count after he sets her free?

She refuses to leave the Count because she loves him; in the end they elope.

The final episode of the adventure takes place on the Isle of Monte Cristo. Maximilien is brought to a cavern where Monte Cristo has prepared a table with flowers and perfumes. Maximilien is ready to die. He sheds a tear for he must leave the Count. The Count gives Maximilien a substance that Maximilien believes to be a death potion. Monte Cristo merely gives him a potion to make Maximilien feel heavy. At this moment Valentine rushes forth to kiss Maximilien. The lovers are reunited at last, fully deserving one another. Monte Cristo leaves them his Champs Elysees residence as well as his house in Normandy. He says that it is Dantes' present to the son of M. Morrel. In this final chapter Haydee also refuses to leave Monte Cristo. They thus sail off into the horizon to elope.

Analysis:

Monte Cristo's final act of divine justice is completed in this chapter. He inflicted ultimate despair upon Maximilien, thus Maximilien is now deserving of ultimate bliss. Monte Cristo is also given a second chance at happiness with Haydee. It is significant that this chapter ends on the Isle of Monte Cristo. It symbolizes and marks the end of the Count's journey of vengeance. The journey had begun here when Dantes had dug up the treasure of Monte Cristo. The journey is now over, and Monte Cristo disappears into the horizon.