My side, based on the scriptures inspired by God.
Man is not the one who first initiated the Death Penalty for those who deliberately killed other humans. According to the Bible book of Genesis, God himself did so.
Actually, for our first parents, Adam and Eve, were warned that if they disobeyed God's law about eating from one particular tree in the Garden of Eden, the penalty would be death.
It's not like they were being refused sufficient food to eat, as they had every other fruit producing tree from which to eat to their satisfaction.
Of course then, before they sinned by their deliberate disobedience, they had been perfect in body and in mind. When they chose to eat fruit from the tree that was the personal property of their Heavenly Father, they perpetrated a deliberate act of theft and and defied God, joining Satan the Devil in rebellion against God and his right to tell them what was right or wrong.
Afterward, God also instituted the death penalty for anyone who purposely killed another human.
Accidental killers, were provided with "cities of refuge," where they had to live until the judges had died. If they left the city of refuge, they could well be killed by the dead person's family. But that same family was not legally allowed to enter the city of refuge to kill the man slayer who was not guilty of murder, but accidental manslaughter.
Since that day, man has always implemented the death penalty for deliberate killers. Nowadays, it seems that criminals have more rights than their victims or their victim's family.
There is also much concern about some who have been found guilty of murder who may have later been found innocent, or who may actually been framed, or through some other miscarriage of justice, whether by witnesses, or through corrupt law enforcement officials, for whatever reason they may have had for withholding important evidence proving the defendant not guilty, or for some political advantage.
Man's laws are not perfect, and neither are their penalties for many different crimes, but that's all we presently have to try to keep ruthless and sadistic people out of society in general.
There are also many penalties that seem to be too lenient for some crimes and way to harsh for other things that may not be nearly so devastating as deliberate murder or some other horrific crimes which too many people are capable of committing.