That’s a great possibility.
The catch would be explaining why Dumbledore killed Narcissa instead of kidnapping her as a hostage. If you’re in control enough of the situation to kill someone slowly and painfully, why aren’t you in control enough to knock them out and carry them off?
But aside from that hitch, that’s a great motive for Dumbledore. Dumbledore does have this pattern of finding “poetic story logic” a convenient excuse for almost unlimited occasional evil. (Harry’s intended parents, Snape as a kid, Snape’s victims as an adult.) And in story terms it would be a great way to finally “open Harry’s eyes” about Dumbledore, if the author wanted to send the story that way.
I’d still say Bellatrix is two or three times as likely a candidate as Dumbledore. Both because Bellatrix fits the boundaries of Harry’s oath to Draco so neatly, and because Dumbledore’s other vile actions have been explicitly in pursuit of assembling “fairytale tradition” on the side of Hogwarts or Harry. Killing Draco’s mother doesn’t fit the pattern for making Hogwarts heroic or Draco a hero, unless...
… well, unless Dumbledore has some specific reason to want Draco to want to try to kill him. Of course, Draco is indeed tasked to kill Dumbledore in canon. But I don’t think MoR!Dumbledore has read his own canon. That would be too much meta.
I think.
I find Dumbledore morally confusing.
His main policy in the fic is inaction.
His reputation in the past of the story is of successful and (by the standards of the wizards) moral leadership. He earned vast respect in the wizarding world without resorting to Lucius’ blood-purist politics or Voldemort’s insistence on slavish obedience.
The few specific deeds he’s done at his own initiative are somewhere between weird and vile. (Jinxing Snape’s relationship with Lily; planning for Harry to have wicked step-parents; etc.)
Largely inactive in the present, good and effective in the reputed past, weird or vile when we actually see him act.
Has there been a leader in real life like this? If this was a real-life person, what would we say about them—that they were a good leader once, and now they’re a crazy one?