Even though canonically, Grindlewald coined in the series, Dumblemore struck me as being modelled after some of the more dubious, even sinister, portrayals of Merlin in some Arthurian renditions.
Well, Merlin
was a Slytherin, in HP canon...
Even in the BBC Merlin series, there are times when he's skirting the Evil part of the alignment chart. One of the prime offending occasions is the 5th-season opening 2-part episode "Arthur's Bane" in which Gwaine, Percival, and a few dozen other knights have disappeared.
When Arthur finds out where they probably are and is determined to rescue them, Merlin hears a prophecy that indicates Arthur would be in grave danger if he went there... so he tries his damnedest to convince Arthur to abandon the quest - abandon saving those 5 dozen knights (two of whom - Gwaine and Percival - are Round Table knights).
In the show's canon, the knights don't find out that Merlin advocated abandoning them to their fate. In the fanfiction, however... there are stories where Gwaine finds out and is absolutely NOT forgiving about this. He's extremely pissed that Merlin - who he considered his best friend (and is actually in love with him to some extent) would just write him off for some vague danger to Arthur. Gwaine's always dropped whatever he was doing to go on some quest to help Merlin, and when he himself needed Merlin's help, Merlin didn't want to offer it.
So the friendship is broken, and in one story Gwaine goes to Arthur and says he's not sure knighthood is something he wants for the rest of his life. Arthur says he never intended Camelot to be a prison, and gives his blessing for Gwaine to leave, if he wants to. Merlin isn't happy about that - it's not long before the upcoming Battle of Camlann, and Merlin threatens to "drag Gwaine back by his hair if he has to". Gwaine figures this is the end. No more friendship; it's shattered beyond mending, he tells Merlin he's leaving with Arthur's blessing, and goes.
Of course I'm a bit biased here since Gwaine is my favorite character, and I hate the way he was killed off in the finale. But I've been taking a critical view of Merlin's increasing obsession with prioritizing Arthur over everyone else, and how he missed or ignored so many opportunities to manage the situation so Arthur
wouldn't be in so much danger.
And the way Merlin ignored his Dragonlord responsibilities toward baby Aithusa is vile.
So to me, Merlin may be the main protagonist, but he is NOT the hero.
And to swing back to Dumbledore, both Merlin (in the BBC version) and Dumbledore kept nattering on about "the greater good" - for Merlin, the greater good was Arthur being king and legalizing magic; nothing else mattered to him, and if friends had to die for that, so be it. Dumbledore sacrificed so many young people, some as young as their first year at Hogwarts, and don't get me started on him leaving 18-month-old Harry on Petunia's doorstop on a frigid October night and not even ringing the damn doorbell to let her know there was a friggin' baby there!