I mentioned the Civil War in Equestria when the Crown of Equestria fought the Lunar Republic.
I also mentioned there was good and evil on both sides, an example of the former being the Eternal Knights of the Lunar Republic.
What do you think an evil character on Celestia's side would be like?

Ah, another ally for the Lunar republic i see. Hmm, I would say, the pupil before Sunset and Twilght and the captain of the royal guard before Shining Armor

The obvious way to go would be a Knight-Templar type who believes any hint of darkness must be purged, but that's possible at any point.

An idea I've toyed with a couple of times is that Luna's marginalisation was planned by elements of the nobility, perhaps to try and weaken enemies that Luna favoured, and the Nightmare Moon incident will have certainly done that, not only to remove Luna's favour, but also cast suspicion on everyone close to her. Of course, no-one saw it coming, but that doesn't stop them taking advantage of it, and it leaves a lot of opportunity to settle old grudges or indulge old prejudices under the guise of war.

And with one half of a diarchy gone and the other emotionally compromised, that leaves a power vacuum, and although most of that was eventually reclaimed by Celestia, there'd almost certainly be many ponies jockeying to take whatever advantage they could of the political instability, and those that are most likely to be successful are going to be ruthless and power-hungry, traits which easily lead to evil actions.

5870890 Ah yes. I've just been explaining to a recent commenter on Intriguing that the Civil War taught the world that alicorns aren't without their lapses. It led to Celestia having to appease the elite for fear of what happened to Rememberly, hence why ponies like the Glasses have so much influence.

5877913 Also led to numerous attempts of outskirt cities like Boulette to seize autonomy.

5870890
How does the name Grand Master Solomon sound?

5910006 Powerful, ominous, and (if I'm guessing their nature right), deliciously ironic.

5910027 Yeah, I've got him as a sort of witch-hunter general who rose to become one of the frontier generals of Royal Equestria, commanding an order of fanatics, keeping his many atrocities hidden from Celestia through his influence in the court and the benefit of being so far from the capital.
He saw the world in black and white and also developed severe pyromania. He subjected every prisoner, soldier or civilian, whatever their age or gender, to Trial By Fire.
Guess how that works.