We don't have enough mythological PEs, so I figured I'd add one to the list, and here's one of the worst dragons throughout the world, among the most sadistic in European tales and the most vile you can find in Spain: Herensuge.
What's The Work
Spanish folklore, specifically Basque mythology, though he can be found in stories outside of Basque Country as well.
Who is He/What has He Done
Herensuge initially doesn't differ too much from the standard fairy tale dragon we all know. Throughout the early 700s and beforehand, he resided in caves on the mountains and every now and then went out to feast on cattle or people. Occasionally he went to the castles of nearby kingdoms or towns to threaten the land by talking to the people or even the kings themselves, demanding them to offer him men or women as sacrifices every now and then (usually one per year) or else he'd lay waste to everything. This is pretty much a standard Tuesday for Herensuge, mind. From story to story he subtly changes but he remains perverse throughout all of them, and powerful enough that almost no one is brave enough to challenge him.
As far as cruelty goes, Herensuge wasn't just a beast out of blood. He had far more than just venom and fire on his side, and while biting someone to death or eating and/or burning them alive was horrible, on occasion he would also just drown them (people and animals alike) or make them go mad with his mental powers until they died, having fun all the while. He takes joy in the deaths of others and that's pretty much his reason for living, and since his stories predate the introduction of benevolent gods like Mari and Sugaar in this mythology, there were very few creatures at the time who were both on friendly terms with humans and who would've been mighty enough to take him down.
There is no given excuse for his actions, he claims evil is merely instinctual and just a natural part of him but as you might expect given his sadistic nature, this is just him messing with people, nothing more.
It's no wonder he's often referred to as a demon or a djinn, who in one story required the help of St. Michael and God himself to defeat. Fittingly, he's sometimes referred to as being pure evil or lacking any goodness in him in many tales.
Freudian Excuse/Mitigating Factors
None, really. There are a few benevolent, honorable dragons and draconic creatures spread throughout the Iberian peninsula, but this is definitely not one of them.
In fact, not only is Herensuge far from one of the more well-behaved or respected dragons in these mythologies, he's by far the worst of the lot. Dragons and other monsters in these lands could attack for territorial reasons, for wealth, for the sake of a princess or other means to bargain, and other similar reasons, but it was generally hunger in a predatory manner and Herensuge was among the few to do it just because he could, the only individual to do so because he was very openly evil (that wasn't from Hell or a servant of the Devil, though Herensuge was an unholy being, but he's still the only openly evil Iberian dragon I'm aware of) and the only one I know of, even within all of Europe, who took joy in making someone go mad until they died, which is particularly twisted.
He was created and used in stories pretty much just to be an evil bastard.
An easy yes, imo.