First, because you aren't, in the case of the murderer, killing someone who is incapacitated. And in this (Rideau) case, far from protecting society, you're actually preventing it from benefitting more from his freedom.
Second, if the only argument for killing someone who has clearly shown every sign of good citizenship since his conviction is that you're doing back to him what he did to another, then all you've got is the most mechanically nihilist corrections system imaginable. It is an expedient that allows you to avoid asking harder questions, like why some people are more likely to be sentenced to death for capital crimes than others, or whether killing is a just response to killing, aside from its ability to soothe tempers.
Christian