There are instances in the Scriptures (Bible, Quran, many others) where followers of God are commanded to utilize the 'death penalty' as a form of almost Godly punishment.
In the Bible, the death penalty was implemented in Israel for certain crimes and those found guilty were tried and convicted on the basis of evidence and the testimony of witnesses. Someone who committed manslaughter (unintentional death) still had to atone for a life lost, but the death penalty was not implemented. There were several cities of refuge where the accidental manslayer could find safety until the death of the High Priest. After which time the offender could go free.
I understand that in the end, the world is to be cleansed of all unrighteousness in order to bring to pass a utopia of peace or what is understood better as heaven, and God is that ultimate judge, yet my question is what benefit has the 'death penalty' brought those in the scriptures?
Where do you get the "utopia of peace" being heaven? In order to fulfill Isaiah 55:11, God will finish what he started in Eden. He designed humans to live on earth forever.....there was never even a suggestion of going to heaven until the Messiah came in the first century. The ancient Jews did not believe in an afterlife because they believed that "sheol" (the common grave of mankind) would hold them until Messiah established his Kingdom on earth, and then call all the dead from their graves. Jesus confirmed this. (John 5:28-29)
The benefit was that the sin committed (usually murder) was atoned for with a life taken for the crime committed, and the assurance that the perpetrator (especially a violent person) would never re-offend. Knowing that the death penalty could be enforced also acted as a deterrent.
I refer to the massacres or conquests of land produced by Jews and Muslims as a commandment of their God?
The only time that God's nation was used as his executioners in evicting wicked people out of the Promised Land, was after their wandering in the wilderness for 40 years following their liberation from Egypt. The Canaanites who occupied that land were gross offenders and Israel was to dispatch them because of their depraved practices.
After that, it was when nations who were eyeing off that good land and wanting to grab it by force, saw God back up the Jewish nation to defend their borders, and to prove superior to that invading nation's gods.
It is written in Mosaic law that an eye for an eye and a tooth for a tooth, a life for a life. Christ attempted to end this seemingly misinterpreted scripture when he said to love those who hate you and do good to those who despitefully use you.
The setting is important to note here. When the nation of Israel were occupants of their own land, they were also under God's Law and he had appointed judges and authorized them to carry out any penalties where those laws were broken.
The Diaspora, (dispersion of the Jews) arose because the Jews were exiled from their homeland by conquering nations—first by the Assyrians, in 740 B.C.E., then by the Babylonians, in 607 B.C.E. Only a remnant of the exiles ever returned to Israel. (Isaiah 10:21-22) The rest remained scattered.....so by the fifth century B.C.E., Jewish communities were found in the 127 jurisdictional districts of the Persian Empire.
When Jesus walked the earth the Jews in Jerusalem were under Roman law, which is why the religious leaders needed the assistance of Rome to execute Jesus. They were not authorized to execute anyone.
Today, there are several places that believe in the death penalty as a form of punishment for heinous crimes, possibly most due to expenses. I don't believe life should be just thrown away by human judgement or expenses as anyone can change their life and start new as we read in many instances in the Bible.
Nations rule by God's permission but they do not necessarily rule by his laws. They have their own. Some do have the death penalty and that is their right. (Romans 13:1-4) If you know the law before you break it, there is no real excuse. The penalty must be paid unless there is clemency at the ruler's discretion. The law doesn't care why you broke it.
How would you justify the Death Penalty?
No one could in today's world. The judicial system is hardly concerned with justice.....it is more concerned with law....and lawyers making vast amounts of money by winning cases.....no real concern about whether their clients are guilty or not.
At the end of this system of things, only Jesus has authority to carry out the death penalty. His justice is perfect, so we need have no concerns about injustice.