I do not dislike Edith; however, I think she deserves all her bad luck up until she voluntarily returns to Downton for Lady Mary's second marriage. It's obvious from the very first episode that she and Mary have a sibling rivalry relationship. Sibling rivalry is not uncommon. They both snipe at each other. Edith's unforgiveable act is writing to the Turkish ambassador detailing Mary's involvement with Kamal Pamuk. Talk about "letting the side down"! This act has far reaching consequences for Mary. It also affects Mr. Bates and Anna. Mr. Bates leaves Downton with his nasty wife, Vera, because Vera threatens to sell the story of Pamuk dying in Mary's bed. Mr. Bates later gives Vera a lot of money in exchange for her promise not to sell the story to a newspaper. Mary is then forced to tell the story to Sir Richard Carlisle, who threatens her with ruin, and Mary feels she has no alternative but to marry Sir Richard even though she does not love him.
I also think that Edith is a hypocrite. She volunteers to help Mr. and Mrs. Drake on their farm. She flirts with Mr. Drake in front of Mrs. Drake and ends up kissing him. Then later, when she finds out that Michael Gregson is married, she tells him she must resign because she finds the idea of a married man flirting with her wholly repugnant.