why did Heraton give Ellen Dean's letter back to Cathy ?

The simple answer is because he loves her. He is a reflection of Cathy and Heathcliff but unlike them he is able to forgive and move on and ultimately have a happy ending.

Heraton does say to Cathy that he has often took her side against Heathcliff although she never knew it; and of course they end up together and happy in the end.

You will need to find your own quotes to support this if you are writting an essay but if you have read the book that shouldn't be a problem. I'd suggest quotes that show the similaritys between the first generation and the second and quotes that support Heraton loving Cathy. Focus more on the second generation than the first.