Introducing Heart Attack, my version of an evil Princess Cadence.
 
I figured since Celestia and Luna have evil counterparts, why not Cadance and Twilight (Not counting Midnight Sparkle, who is technically Sci-Twi’s evil counterpart). I knew it was going to be a bit of a challenge, since Cadance and Twilight are more so based on concepts and emotions rather than celestial bodies, but I did my best so far.
 
With Cadance the only thing I could think of when it came to the darker side of romance was lust (Hence her bedroom eyes). I know that sounds very NSFW, so I tried to keep it as PG as I could. I wanted her colors to feel enticing, inviting, and warm, but I also added in some black and some dark shades to indicate a clear deviousness. Her face had to give off a very “Come hither” expression, with a distrustful undertone.
 
Heart Attack, unlike her good counterpart, is very promiscuous by nature. She is seductive and is able to use her charms and charisma to get what she wants. Similarly to a changeling, she can absorb love from anyone. And the more she absorbs, the more powerful she gets. She can also entice creatures to fall in love with her by singing, though this only works on males.
 
EDIT:
 
I changed her name.  
Heart Attack sounds bad, but more threatening and original.