"An eye for an eye would make the whole world blind." -- MK Ghandi

Natural isn't necessarily good, and revenge isn't a natural instinct to all. Furthermore, a revenge ethic perpetuates through society in such a way as to prevent peace and conciliation. Many murderers feel that they themselves are meeting out justice.

Far better, as Jesus attempted to do, to nurture an ethic to forgive. This is implausible in the face of murder, but we can reduce the glory of murder thorough a dismal life imprisonment.

Those who kill often feel that those they kill have wronged them, and feel a natural inclination toward revenge. Should we not thereby also respect their "natural" motives, by your logic, and therefore not punish any killer at all unless they murdered a complete stranger in cold blood?