So question: how can Christians support the death penalty since the Prince of Peace

[PeaceNikki]

20. That's not exactly true. It really is "You shall not murder"

(Hebrew לֹא תִּרְצָח lo tirṣaḥ )

Murder is different than kill.

http://en.wikipedia.org/wiki/Thou_shalt_not_kill

According to the Priestly Code of the Book of Numbers, killing anyone outside the context of war with a weapon, or in unarmed combat, is considered retzach, but if the killing is accidental, the accused must not leave the city, or he will be considered guilty of intentional murder. The Bible never uses the word retzach in conjunction with war.

The act of slaying itself, regardless of questions of bloodguilt, is expressed with the verb n-k-h "to strike, smite, hit, beat, slay, kill". This verb is used of both an Egyptian slaying an Israelite slave and of Moses slaying the Egyptian in retaliation in Exodus 2:11-12. The Covenant Code and Holiness Code both prescribe the death penalty for people that commit n-k-h.

Another verb meaning "to kill, slay, murder, destroy, ruin" is h-r-g, used of Cain slaying Abel in Genesis 4:8. When Cain is driven into exile, complaining that "every one that findeth me shall slay me" in Genesis 4:14, he again uses this verb (h-r-g).

The commandment against murder can be viewed as a legal issue governing human relationships, noting that the first five commandments relate strongly to man's duty to God and that the latter five commandments describe duties toward humans.[9][10] The commandment against murder can also be viewed as based in respect for God himself. Since man is made in God's image, the shedding of innocent blood is viewed as a direct offense against the Creator.

They believe it is justified killing due consequence for crime.

The Torah and Hebrew Bible made clear distinctions between the shedding of innocent blood and killing as the due consequence of a crime. A number of sins were considered to be worthy of the death penalty including murder,[21] incest,[22] bearing false witness on a capital charge,[23] adultery,[24] idolatry,[25] having sexual relations with a member of the same sex, etc.[16]

For example, the Exodus narrative describes the people as having turned to idolatry with the golden calf while Moses was on the mountain receiving the law from God. When Moses came down, he commanded the Levites to take up the sword against their brothers and companions and neighbors. The Levites obeyed and killed about three thousand men who had sinned in worship of the golden calf. As a result, Moses said that the Levites had received a blessing that day at the cost of son and brother.[26] On a separate occasion, a blasphemer was stoned to death because he blasphemed the name of the LORD with a curse.[27]

The Hebrew Bible has many other examples of sinners being put to death as due consequence for crimes. Achan is put to death by Joshua because he caused defeat of Israel’s army by taking some of the plunder and hiding it in his tent.[28][29] David ordered that an Amalekite be put to death because he claimed to have killed King Saul.[30] In his charge to his son Solomon, King David ordered him to deal with the bloodguilt of Joab, who had murdered Abner and Amasa.[31] Solomon ordered that Joab be killed:

Strike him down and bury him, and so clear me and my father's house of the guilt of the innocent blood that Joab shed. The LORD will repay him for the blood he shed, because without the knowledge of my father David he attacked two men and killed them with the sword. Both of them—Abner son of Ner, commander of Israel's army, and Amasa son of Jether, commander of Judah's army—were better men and more upright than he. May the guilt of their blood rest on the head of Joab and his descendants forever. But on David and his descendants, his house and his throne, may there be the LORD's peace forever.

—1 Kings 2:31-33 (NIV)