I can believe Rick's wife reacting that way. She's been pretty unstable for a while — perhaps from the beginning. In my opinion, she was intentionally leading Shane on in several scenes in the last half-season. (One that comes to mind is the scene at the windmill, but there have been others.)

But yeah, Rick snapping like that at the end didn't feel right at all. I honestly have the feeling that it was handled wrong by the actor and the director. It came across as overbearing. Assume the same lines were delivered by someone who felt that trying to be accommodating to the group's various factions had not only gotten a lot of them killed, but forced him to kill his best friend. Not someone who suddenly became overbearing, finding fault with everyone else, but someone grieving and finding fault with his own actions.