Have you read the latest chapter of Fairy Tail? Chapter 241? If you haven't it's time to read it since you will find out the reason behind one of Fairy Tail's most notorious villains obsession to complete the Great World of Magic. Yes, you heard it right! Ultear Milkovich, the truth revealed at last!
Story (Spoiler Warning):
Due to the excess Magical Power that her daughter possess when she grew up, Ur was forced to carry her to the hospital. However, the doctors sees Ultear's potential and tells Ur that she died. While Ur grieves for the lost of her beloved daughter, the doctors cure Ultear who wakes up and thinks that her mother abandoned her. The doctors experimented on her, trying to amplify her Magical Power. One day, Ultear escaped and returned to her home only to find Ur, happy, with Gray and Lyon. Ultear vowed to never forgive her mother and returned to the research lab. The doctors manage to finish their experiment and Ultear destroyed the lab and set out to get revenge on her mother. One day as she is researching about Zeref, Hades appears to her and tells her that she would be able to travel through time with Zeref and the Lost Magic, the Arc of Time. Ultear vows to complete the Great World of Magic in order to travel back to time and reverse her cursed existence. The only thing she wanted in the world was more time with her beloved mother...
If you still hate Ultear after that, go ahead. Ultear's story just shows that her love for her mother would make her do evil things just to get what just a little more time with her.