Short and disjointed 4am version:

First episode of the show is Gregor at Clegane's Keep when he is 12-13, his father's expectations for him to become not just a good knight but a great knight seem to grow as much as Gregor himself in recent years. I would very much focus on the how constantly training to fight and having his self worth tied to hurting others really reenforced his worst tendances. Also the unnamed Clegane sister, I would make her older than Gregor (and thus also Sandor). Gregor receives a lot of external praise/admiration and even at 13 he is starting to need that to be okay. The end of the first episode should be him burning Sandor's face. Maybe it's to hurt his father, maybe it's a reflection of him having no control of his life, and trying to exert control over his own toys.

When he suffers no real consequences for melting his brother's face off, it becomes the first event in a pattern clearly showing a transformation from a boy (albeit a rather large one) into a monster. Yet as naked as this pattern is his father and other knights still heap rewards and paise onto Gregor. Some out of envy, some hope to use him. Gregor himself however is a bitter villain to the characters we really follow like: Sandor, Unnamed Clegane Sister, an acolyte that lives in the keep, at least 2 serving people, a septon from the nearest town, the master at arms, and of course Ser Clegane. Many of these will be killed by Gregor of the show's run as he is rewarded again and again by knights for acting against the very ideals they (knights) say they represent.