I was a bit disappointed by Carol's exit I have to say. I thought a big character like her deserved a better ending... but I suppose at least the door is open for a return.
That said... I think the problem that a place like Alexandria poses is that the imminent threat is sort of removed on the surface. They live in houses, have families, choose to get pregnant, make meals, have walls etc They are allowed to retain/regain their humanity to a certain extent. For intelligent women like Carol who have been pushed to extremes like she has, this means she has the chance to reflect on who she has become. She wasn't a soldier, she was a downtrodden, abused housewife who has now called perhaps 20 people. I can see how that would be a lot to come to terms with.
People like Rick, well, he was a Sheriff, used to being in charge and having to make difficult decisions. There was probably a certain amount of black and white to him anyway. Daryl was used to living under the thumb of his brother, and although obviously Rick is different, Daryl does still allow Rick to make the difficult decisions for him and just follows suit as it is usually for the good of the group. The rest of them (from what I can recall) haven't had to be as brutal as Carol.
And to be fair, having to kill Lizzy after knowing she killed her sister... that would have finished me off. So fair play to Carol for keeping it together for so long.
I just wish that it wasn't her that had to go. I had massive respect for her.