God is the author of life. By His word, He effortlessly spoke into existence all forms of life, except human life. Because humankind was to be after His own image and likeness, He so specially created human with his very own hands and He imparted His very own likeness into man by breathing His Spirit into man, and human life ensues.
If we are willing to have the humility to grasp this truth, then we would deeply realize that before God, we really own nothing -- not even our very "own" life!
If -- by the account of the countless most wicked and evil things that people have done -- God's most beloved Son Jesus had paid with His very own life the retribution of all of the sins (past, present, and future) of all of mankind (past, present, and future), then what greater power or virtue there is that would make us worthy to take the lives of worst criminals or sinners?
God's justice is never without mercy. In fact, His justice exists because of His mercy. Even hell exists because of God's mercy! Hell's primary purpose is not as a deterrent for mankind from committing sin. It is primarily intended as the eternal abode for souls who, despite knowing God (the Way, the Truth, and the Life), remain perpetually rejecting Him with teeth-gnashing rejection. Hell is the eternal place for voluntary (or free will) perpetual separation of one's self from God. Hell is not a retribution, rather it is God's mercy that is still at work despite a soul's utter rejection of God. Nothing can separate a soul from God's love, not even hell.
Before God's Son paid on the cross with His life the retribution of mankind's sin, the penalty of sin then even included death in public (by stoning). But after the cross, sin is met with God's mercy that gave the hope of salvation through repentance in the name and sake of Christ's redemptive suffering of the retribution of sin which was death.
So, what makes us think we are worthy to usurp the ownership of human life? Are we worthy to have the Author and Owner of human life be under our authority?
Lord, I am not worthy to have you enter under my roof, but only say the word and I, your servant, shall be healed of my own sinfulness!
"There is no fitting punishment without hope! Punishment for its own sake, without room for hope, is a form of torture, not of punishment." -- Pope Francis