What Carol did was wise, pragmatic, very practical and utterly wrong on every level. Whether her attempt to contain the infection was successful or not is immaterial. She took it upon herself to euthanize two helpless people in their sleep. How can anyone possibly trust her knowing that she could something like that again? It wasn't her call, it wasn't her duty, it wasn't her responsibility. Ironically it's a similar attitude to the one Shane had when he wanted to call off the search for Sophia. "Someone has to step up and do the hard thing" or words to that effect. Except what he wanted and what Carol did was just the opposite. It was the easiest thing in the world. The quick fix. The simple solution. For all Carol's talk of not being afraid and never giving up, she's the one that acted purely out of fear. Those people could easily have been rendered harmless even if they turned. They are in a prison, no? Those doors have pretty sturdy locks and I'm sure there's still a whole lot of cuffs and leg irons all over the joint. It doesn't take much imagination to see what kind of precautions could have been taken.