If one is a Christian or a Jew, or even just a general theist, you shouldn't feel guilty wishing the death penalty on proven murderers.
1And God blessed Noah and his sons and said to them, "Be fruitful and multiply and fill the earth. 2The fear of you and the dread of you shall be upon every beast of the earth and upon every bird of the heavens, upon everything that creeps on the ground and all the fish of the sea. Into your hand they are delivered. 3Every moving thing that lives shall be food for you. And as I gave you the green plants, I give you everything. 4But you shall not eat flesh with its life, that is, its blood.5And for your lifeblood I will require a reckoning: from every beast I will require it and from man. From his fellow man I will require a reckoning for the life of man.
6"Whoever sheds the blood of man,
by man shall his blood be shed,
for God made man in his own image.
7And you,[1] be fruitful and multiply, teem on the earth and multiply in it."
8Then God said to Noah and to his sons with him, 9"Behold, I establish my covenant with you and your offspring after you, 10and with every living creature that is with you, the birds, the livestock, and every beast of the earth with you, as many as came out of the ark; it is for every beast of the earth. 11I establish my covenant with you, that never again shall all flesh be cut off by the waters of the flood, and never again shall there be a flood to destroy the earth." 12And God said, "This is the sign of the covenant that I make between me and you and every living creature that is with you, for all future generations: 13I have set my bow in the cloud, and it shall be a sign of the covenant between me and the earth. 14When I bring clouds over the earth and the bow is seen in the clouds, 15I will remember my covenant that is between me and you and every living creature of all flesh. And the waters shall never again become a flood to destroy all flesh. 16When the bow is in the clouds, I will see it and remember the everlasting covenant between God and every living creature of all flesh that is on the earth." 17God said to Noah, "This is the sign of the covenant that I have established between me and all flesh that is on the earth."(Genesis 9:1-17)
This is called the "Noahetic Covanent" and since it was with Noah, ancestor of every human alive today, it applys to everyone (unlike later covenants which applied only to Israel). It has the seemingly ironic quality in that it clearly establishes the reason for the death penalty is just BECAUSE humans are made in the image of God. (think of that pro-lifers!) To destroy God's image is such a henious act that God commands (not merely allows) that governing authorities execute anyone who would do such evil. In no case does this more obviously apply than with Mohummed and Malvo.
(By the way, this covenant, in the verses immediately preceding the underlined part, also clearly allows for the eating of meat--so one should never let vegetarians act as if they are morally superior--we have God's word on it that they are not.)
If you are tempted to think, this was so long ago, and it doesn't apply to us in such modern times...ponder the rainbow, which says it does.
And wasn't it God Himself who gave all of us a death sentence in that same book of the Bible?