Throughout various stages of my life, I've known someone who could only be described as an asshole. I mean, we all know assholes. But I'm not talking about just any old asshole, but someone who reaches a higher assholian level, a kind of nirvana of the asshole class. [I'm using asshole in the gender neutral way- can substitute 'bitch', 'dick', 'wanker', etc...] What separates these individuals from the run of the mill, bread and butter, average assholes, is their propensity- their deviously underhanded and immensely disturbing tendency- to periodically do something decent. And then someone says 'wow, deep down, he's a really nice guy.' So they enjoy spending 99.9% of their waking time (and I imagine that in their dreamtime they enjoy being assholes) as complete assholes. But then, they reap the fruits of a 0.1% period of decency. All the while, someone who spends 99.9% of their time being decent, will occasionally slip up, and then someone invariably says 'wow, deep down, she's an asshole.'
It's not right, but that's how the world works.
And I think, that's the way it works with fictional characters, as well. The most interesting characters are ones who we think we've got pegged, and then they surprise us. Both those who are dark, with decent moments, and those who are light, who turn out to be darker than we could imagine. One dimensional characters are a bore- no one in real life has only one dimension. Neither should a character.
So, just be an asshole.