What happened to the kid from season 1?

Okay, he wasn't exactly a child prodigy, and I do remember some people criticising him back then as well. But one thing I liked about him was that he looked innocent and naive, and that's something I expected from the character. He is soft hearted, and easily manipulated. I'm just wondering what happened to that kid, and why did they replace him with the worst child actor in the world. Worse than early Carl Grimes. It's like they looked at the auditions and picked out the most spiteful little boy who glared at people sideways. They must have thought "yep, this kid can portray hate. and that's exactly what we need for that one scene in the finale".