Carol had to go because she wasnt able to interact with a group anymore. It's like how Michonne was--she had to relearn how to be with people. Carol lets go of the group and distances herself (hopefully only temporarily) and Michonne lets go of her vengeance and is smiling--they play off the same theme. And Carol didn't really argue too much, she knew she didn't belongs anymore--at least not for now.
Now, with the Governor back in the picture...?
The walkers were fed by someone inside the prison--nothing has shown the Governor has been inside during this time. Though he may have been leading the walkers there.
I think Rick's estimation of Carl just went up, and he seemed to like that he and Carl could work together.