These people are trying to build a community that maintains the standards of civilized society yet you think they should just go around murdering members of that society with impunity? Carol taking it upon herself to be an Angel of Death is justifiably frowned upon within the societal structure that was set up within the prison.and this is why i feel carol was jusified. put them out of their misery and keep it pushing
And Rick banishing her is a total leadership decision because he weighed the options of sending Carol away or bringing her back and having the reality of what she did tear their new society apart. If he brought her back there would be the issue of Tyreese wanting vengeance, a culture of distrust being established at the prison, deciding if she should be punished which would possibly tear the group apart with different factions choosing sides. It would have been a mess.
Rick chose the option that would be less destructive to the group.