There is a code to living that cannot be violated. If we live in a civil society, we all have a personal mandate to be civil. Laws exist to act as a hedge against evil. They don't always work. There are some brands of evil that need to be purged. Everyone knows there are people who are so evil they need to go. The Mansons, Escobars, McVeighs, John Wayne Gacey (spelling?) and all those types.
A civil society breaks its pact with law abiding people people when it allows these people to go on living when they've not only broken the social compact, they've destroyed it by dint of murdering, raping, selling drugs, ruining the lives of others, some of whom were parents who have left a child parentless, sometimes orphans, who then become a burden on the state (albeit a worthy burden). Actions like these are grounds for state-sanctioned execution of the offender.