Carol at this point is one of my favorite characters now, easily.
Lizzie had to go, I don't see why there's any question about it in here. Hard choice? Sure. But it had to be done. Not only did she kill her little sister in cold blood, but her plans wouldn't have stopped there. In her mind, she thought being a walker was a better life. How soon before she'd try and kill Judith, Tyreese, Carol and then herself? Hell, she would have killed Judith if Carol hadn't shown up.
I loved Carol's reasoning at that part too. That it'd be unwise to murder Judith considering she couldn't walk yet. Had she panicked like some of the other characters undoubtedly would, Lizzie might have gotten knife and/or trigger happy. Everything Carol did in this episode made perfect sense in my opinion. I also LOVE the irony that Rick cast her out. Didn't he say something like he wouldn't want his children (Judith and Carl) around her to her face? Well, turns out Carol saved his daughter and is protecting/watching her with Tyreese. I'm hoping we get to see Rick's reaction to this.