There is a technical difference between killing and murder. The taking of a human life, in some cases, is justified (not murder). In Exodus 20: 13 we read, "thou shalt not kill." However, those under Moses' Law (theocracy) who committed capital offenses such as murder (unlawful taking of a human life) were to be "surely put to death" (Ex. 21: 12).
The origin of the death penalty. "Whoso sheddeth man's blood, by man shall his blood be shed: for in the image of God made He man (Gen. 9: 6). God ordained the death penalty. God also appointed "man" to execute the death penalty. In Romans 13: 1-7 the "sword" belonging to the civil powers is not to be born in vain (vs. 4). Those who oppose correct capital punishment are opposing God who instituted and commands it! If the death penalty were inherently wrong, Paul would not have offered to freely yield to it. Hear him: "For if I be an offender, or have committed anything worthy of death, I refuse not to die..."(Acts 25: 11).
Beloved, the death penalty, when correctly exercised, is not murder because God legislated capital punishment. As man seeks to remove God from right and wrong and morals in general, we will continue to call evil good and good evil (Isa. 5: 20). The very concept of "punishment" seems to have departed from the thinking of many (please carefully read Romans 13: 1-7). Surely we do not question the "morals" of God!
source :
http://www.biblequestions.org/Archives/BQAR237.htm