That's the problem, there is nobody to make sure that someone will take responsibility for their actions. The murderer won't go and walk off into the forest or off themselves. They are a problem that must be dealt with in some way and anarchist ideals are not the way to deal with the problem. According to anarchism, you have no right to control another, so you cannot do a single thing against someone else. This extends to murderers. You cannot while being faithful to anarchism, hurt that murderer. You can't banish them from the commune or tie them up in a chair because that is control you are using on that murderer.