I disagree Cersei's Season 6 story was fan service.
But if it was, they at least laid the foundations for it, and her behavior was consistent with the character
At the end of Season 5 Cersei was released pending further charges, but if she been found guilty of those charges she would have been executed. There is no way Cersei would let that hapen.
Not a problem since her champion, the Mountain, would almost certainly defeat anyone else in trial by combat
The Sparrow finally gloms on to the flaw in his plan when he sent Lancel and a bunch of other sparrows to take Cersei back to the Sept. Lancel says there will be violence unless she comes peacefully. Cersei refuses, one of the Sparrows attacks the Mountain and is promptly gutted like a fish
Shortly afterwards Tommen, no doubt prompted by the High Sparrow, prohibits trial by combat (convenient no?)
Now Cersei is backed into corner. This is when she is most dangerous and most ruthless (such as what happened to Ned Stark in Season 1).
Cersei asks Qyburn to investigate a rumor she heard (from Jaime). Qyburn tells her, onscreen, that the rumor is true.
Then comes the destruction of the Sept. Getting rid of the Tyrells and Uncle Kevan was just a bonus from her perspective.