We are talking about a few individuals who did a great deal of damage to society and if turned lose again will repeat their actions.
How do you know these lives are worth taking? How do you know these lives are of no value? How do you know that the idea of damage won't be defined and redefined again by faceless bureaucracies until it reaches a point where it lets loose in society and a "few individuals" just need to get murdered for the "good of everyone"?