@Mugen Kagemaru
 
well, the elements were like “bam!” and suddenly luna’s like “oh hey I’m luna again.” why should they believe the elements of harmony are capable of just poofing someone good? they never did that before! so it’s not insane troll logic, but it is hard to disprove.
 
the way not to disprove it is to form a secret splinter organization, hiding behind the scenes and trying to stop the evil plans of nightmare moon.
 
what they should do is form a secret splinter organization hiding in plain sight, and ruthlessly spy on her. and only when you catch her off guard loudly monologing in song form marching around the castle at midnight, do you conclude she’s still black snootie.
 
and when your spies say, “eh, the most evil thing she did was frown at a charity drive,” you don’t assume they’re under her evil spell, knock them out and tie them to a tree, then go slaughter some village just because they don’t like the day as much as you do, therefore totally support all of the nightmare’s plans.

Derpy: Princess Celestia, my brother’s part of a evil cult!
 
Celestia: I’m sure this cult is just misunderstood. I’ll meet with their leaders and settle this peacefully.
 
Derpy: They want to murder your sister.
 
Celestia: SHOW ME WHERE THEY ARE SO I CAN TEAR OUT THEIR SOULS!

Interesting. What a clever way to make them evil in this.

“Yes, we’re crusaders. What’s your…”  
“…”  
“There’s what?”  
“…”  
“Cutie Mark?!”  
“…”  
“Well we’ll see abut that.”

@Rated Ponystar
 
Honestly my guess came more from how they’re convinced Luna’s still Nightmare Moon.
 
The conspiracy from Death believed Nightmare Moon killed Luna and was, even in the Present, using her body as a host, and Luna’s Return was basically just Nightmare using a smokescreen while she got her powers back.
 
A load of Insane Troll Logic but still…

@Mugen Kagemaru  
Sorta, Princess Luna is a big target of theirs, but their overall goal is the extermination of all the creatures of the night such as werewolves and vampires. Since Luna is a vampire, 1/4th, she’s on their list

So basically the conspiracy from The Death Of Princess Luna except expanded into a splinter organization and, assuming I’ve got this next part right, has been a thing since before Luna’s Return, whereas in Death I think it was formed after Luna’s Return.