One of the 10 commandments that God gave to Moses was "You shall not murder." The act of taking the life of another without justification is abhorrent to the Lord. We are all man in god's image and our holy father intended for us to live together in peace. We are supposed to feel love for our fellow man not anger and hatred. One of the first cases of murder mentioned in the bible is Cain and Abel, the sons of Adam and Eve. While out in the field Cain murdered Abel out of rage and jealousy, and then tried to hide his actions when asked by their father where his brother was. Cain was jealous because Abel's sacrifice was accepted by the Lord while Cain's was ignored.
God placed a curse on Cain because he murdered his brother, but Cain was not repentant so God also placed a mark on Adam and Eve's son so that everyone would know the horrible crime that he had committed. Many mistakenly believe that this commandment means that capital punishment is wrong but there is a distinction between justified killing and murder, and even the bible recognizes this distinction. Because we are made in the image of god killing another human is like killing god. Murder leaves a stain on your soul, one that can never be removed. Our heavenly father is merciful though. Even those who have committed murder can be forgiven if they are truly sorry, they understand the error of their ways, and they ask god for forgiveness.