I've been kind of bouncing around the idea of a villainous Cadence for a while now, mostly as the subject of a humor story...
However, speaking with a member of this site (Veltrusen) made me realise that... a genuine Tyranical Cadence, whether as an evil empress of her own kingdom, or in flat out Nightmare form would be... pretty terrifying.

So... I thought I'd pose the question to you guy's. If Cadence were to become a nightmare, or become a villainous empress, how you you think she would act?

It depends on where her darkness comes from.

Nightamre Moon in most stories is manifestation of Luna's jelousity or loneliness, occasionally an evil spirit that possesed her. Evil Celestia is most of the time a manipulative tyrant who want to control everything, which came from taking her manipulative nature in the show, and turning a "throw at deep water to teach how to swim teacher" into "tyrant or mastermind or just troll."

Twilight turning evil mostly comes from her desire for knowledge, having an evil teacher, being possesed or finding herself being manipulated and put in danger in each step, which push her into rebelion or facing her teacher.

I think evil Cadence need to be related to canon Cadence in some way, and I have a few suggestions for this:
1) Pain of lossing a loved one. Either Cadence outlive Shining Armor or other pony she loved, and this push her into despair, or Shining Armor die on her very eyes (probably from Chrysalis). If you go for high drama, have Celestia send Shining Armor into a suicude mission or one of alicorns killing him for some reasons,

This Cadence would either turn to necromancy in attempt to bring the loved one back to life, or become depressed and bitter. She would be afraid to love or care of anypony in fear of being hurt when they die. This Cadence isn't evil, but rather desperate or lonely. The exception is if she is trying to avenge the one she lost. In this case, she will not try to hurt anypony, especially those she view as friends, but she would go far to achive her vengence.

2) Sombra take control of Cadence and turn her into dark empress ( similar situation to final boss of Paper Mario 2 the thousand years door). In this case she would either try to resist Sombra and help her firends in freeing her while he control her body, or he would try to corrupt her with lies, amking her face many dilemas and hard choises.

3) Nightmare Moon try to posses Cadence to have her vengence, only to find out that her vessel have diferent ideas. It would end up with either Nightmare Moon redemption as Cadence show her kindness and compassion she desired so much, or one of Cadence's adentures alongside Twilight where Nightmare Moon would see everything from a diferent perspective. Not from the one who try to bring enternal darkness that main 6 try to face, but from ally of Twilight who try to protect Equestria from another villain.

4) Cadence was alive for over thousand of years, and after crystal empire was taken over, she ended up as a prisoner and tortured. Sombra would try to turn her into his minion, but he either fail to get Cadence to serve him, or she become his servant, but he fail to complitly eliminate kindness from her heart. In here when Sombra is defeated, Cadence can escape and try to survive (she would go renegate as her mind was messed up by Sombra, or just a harsh alicorn ruler who saw to much evil, and is now unsure where she is crossing a line between good and evil.

All in all, I think she would act with hesitation because of her kind nature, unsure if what she is doing is the right thing to do. As for the rest, it depends on what influenced her to turn evil as I stated above.

This is all I got. Sadly I failed to think of anything comedy related. Hope it helps anyway.

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Oh! Sorry, should have clarified; I'm not fishing for ideas, just sort of curious about how an evil!Cadence might work in a story, in general.
(I have my comedy take pretty much nailed down- was just curious what a genuine evil Cadence might be like, according to her fans.)

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Well, it depends on what influence her. She would act diferently if possesed, diferently if she lost somepony she loves, diferent if she was betrayed by those she trusted. Long story short, I need to know why Cadence is evil in the first place before I can tell you how in my opinion she should act.

I can help more if you tell me what will cause her to turn evil in your story.

4583973 empress of pure lust. Always wants to buck somepony.

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I agree with CommanderX5

Its all about influence, In my opinion the best one to go with would be loosing her husband for some reason or another....

Honestly I think its a genius idea.

Two Words: Pony Slaanesh