I had no problem with Luke.
This is an guy who was constantly being told by everyone that he was the big hero. he was the savior. It was his destiny to be great. Then, when they needed him the most in a single moment of doubt he messed up and it all came crashing down. He made a mistake and it broke him. he pushes his sister and his best friends' kid down a path of darkness.
I can easily understand why he'd become a cynical hermit after that.
His comeback at the end though, that was completely badass. I wish we would have gotten a little bit ore of that, but I can't complain too much.