I stole this from the internet (slightly edited to avoid spoilers),
Lying awake one sleepless night, this troper had the following epiphany: Littlefinger is The Hero of the story. I'll say it again. Petyr Baelish, AKA Littlefinger, is the hero of the story. Or at least he would be, if this was a traditional fantasy story. Think about it: he starts out the heir to basically nothing, but is still forced to spend all of his childhood associating with the super powerful, most of whom refuse to even call him by his given name. Also, he has the disadvantage of being small and weak in a society that values strength above all else. Despite this, he manages through his own cleverness and force of personality to launch a successful career in politics, eventually becoming one the kings most trusted advisers. And just to increase his woobiness factor, lets say that not only does he fail to get the girl, but she goes off and marries someone she's never even met. And then his rebound girl gets married off to someone she's never met. Now, if this were a traditional story, Littlefinger would find a way to overcome his adversity, win Cat back, and be recognized by the nobility as the valuable ally that he is. Except this isn't a traditional story, so Littlefinger's victory is pyrrhic at best; no matter how valuable he is to the nobles, they'll never consider him to one of them; he'll always be an outsider. And even his material success in rendered meaningless by the fact that the women he loves isn't at his side. Soon, his disillusionment turns to bitterness, and then his bitterness becomes hate. And then he decides that if he can't get his happy ending, then no one will. And thus, the best villain ever was born. This also explains his relationship with Sansa Stark. As is explained above, Sansa is meant to represent the traditional fantasy heroine — i.e. someone who believes in happy endings, like Littlefinger used to before he became disillusioned. He sees a bit of himself in her, and that's why he actually gives a bleep about her, something he is pretty much incapable of doing for anyone else. So in short: the deconstruction of The Hero is, in fact, The Big Bad. GRRM is a fucking genius. —Taelor
Adding to that: if this were a traditional story, Cately would still love Littlefinger, and the best Ned Stark could get from her would be respect, so when he would get out of the way (probably by banishment), she would gladly fall in Littlefinger's arms - and since he is now a noble and thus suitable they would happily marry and this would be presented as Littlefinger ultimately overcoming all (and presented as Happy Ending). Except here Cately came to love Ned (as quite probably in such circumstances), and then he was executed rather than banished, (since the great Chess Master Littlefinger simply didn't account for Joffrey's brutality), and Catelyn went mad with grief as a result, rendering the whole thing into an ultimately futile Moral Event Horizon for Littlefinger.