Jesus said ""Love your enemies; and do good to those who hate you" to persons trying to lead moral lives, not to the State.

The death penalty, as with all laws, crimes, and penalties, belong to the category of things that are Caesar's. The role of the State in traditional Catholic teaching is quite different than an individual's role.

An individual cannot take upon herself or himself to decide a law, apprehend violators, and inflict punishments.

If the victims you mention grabbed Roof and strangled him, that would be vengeance, which is morally prohibited.

The reason why other First World countries have no death penalty is the same reason they have abortion on demand: Man is the center of everything and this life is all that counts, so grab it all and satisfy yourself. Do what feels good.