Vampires often brood. And they can have mood swings, just like the rest of us.
But there are cases where a vamp is just bad news, and Niklaus Mikaelson is one of those. He's a Sociopath. Okay, that's not technically what we call it, but you still need to cross the street when you see him coming.
Antisocial Personality Disorder is no joking matter. Deception, irritability, a complete disregard for others and an utter lack of remorse are bad enough when spread out over a lifetime. But over an eternity? Better duck.
By century two complex dangerous sub-characteristics start crystallizing: like belligerence, and the obsession with one's own infallibility. And the vengefulness? Hell, how many times has he staked his poor sister?
To make matters worse, he's a werewolf hybrid. Add fearlessness, recklessness and zip impulse control to the mix.
The genesis of this disorder is believed to be trauma. You know, like when your mom turns all your siblings into vampires. There is no hope for poor Klaus, and less hope for any living thing he rubs up against.
Fortunately, he has the spin-off to keep him occupied. But if it's canceled, better to get out of Dodge.