I think Finn was actually an extremely well-written character (especially in light of Kim saying Finn was, to a certain extent, intentionally meant to be received by us in the way he was). Totally despicable and self-centred and obsessive and ultimately not cut out for the world he was thrown into, but well-written.
Because he’s real.
People just like Finn actually exist. They pretend they’re a Good Guy and a lot of the time, we believe them, but that’s just not the case and eventually all their bad qualities start to come out and reveal themselves and at the end of it all we’re left shaking our heads wondering - just like Clarke - how we could be so short-sighted not to have noticed it right from the beginning.
I think that’s part of the reason why so many of us hate him so much. He’s not a typical villain. He’s not even an anti-hero. He’s that guy you know. The one you thought might actually be different. He’s that guy you thought you liked … the one who turned out to be everything he told you he was not. He’s the guy that broke your heart, and the entire time tried to pretend he didn’t by covering up the wound with a flimsy little bandaid that falls off with each movement.
And then does it again, all the while sporting the complete and utter lack of self-awareness required to wonder why your cut is still bleeding.
Finn’s so real, so realistic, so plausible, that it’s honestly unsettling. He’s the kind of character that makes you wonder how many Finn’s are wandering around in your life right now that have yet to reveal themselves.