To understand that, you need to realize I started writing my story way before Keep Calm and Flutter On and that I will not be reforming Discord.
That being said, with giving away too much, Cadence was Discorded by Discord because it is all part of his plan that was put into motion prematurely by the arrival of an unexpected catalyst. He did so because she was essential due to her position both geographically and in terms of her being a head of state. Her influence and resources are prime for his endeavors.
You don't even have to read my prior stories to see how important Cadence is to all of these villains (with the exception of Lightening Dust.)
She's in the Crystal Empire therefore, she has access to the mirror to get Sunset, She can strike a deal with Chyrsalis because her kingdom is practically run on love, more so than enough to feed her horde. She's located in the Frozen North where Sombra is, plus Sombra is directly linked to the Crystal Empire. The Crystal Empire is by-and-far a great distance from Canterlot and therefore not under Celestia's immediate gaze. These are all great reasons for Discord to want control of Cadence.
I think Cadence could be the single greatest tool Discord could bring to bare against Celesitia, and I think he could do it easily. It also fits with why in my story she is an alicorn.
Cadence is evil because plot dictates that she should be, not because, "I don't like her".