God not only allows for the death penalty, but specifically prescribes it--and it is not because life is not sacred, but because it is.
“Whoever sheds the blood of man,
by man shall his blood be shed, for God made man in his own image. Genesis 9:6 ESV
Specifically, if someone murders another, his crime is so heinous that God declares he has forfeited his own life. The emphasis is on the sanctity of the life he took--not on the life of the taker. He has sinned against God in doing so, and it is God who requires the penalty be paid. It is important to note that this law precedes the Law of Moses and is thus not part of that covenant. It is, therefore, fully applicable today.
Secondly, in the law of Moses, God also prescribes the death penalty for rape. Again, it is such a heinous, horrible, despicable offense against the sanctity of the life of the woman, that God punishes the crime severely. It is, again, that God holds the life of the victim as sacred that He requires the forfeiture of the life of the perpetrator. This law is within the Law of Moses which was part of God's covenant with the Jews, so this could be seen as a pattern to follow rather than a prescription to institute in any nation.
Thus, there is no contradiction in supporting capital punishment while being against abortion. Both positions recognize the sanctity of life. The Pro-Life position recognizes that the life of the innocent child begins at conception and deems that no one has the right to end this life.
Capital Punishment recognizes the sanctity of the life of the victim and sees that taking the life of an innocent person is such a grave offense, that the highest of penalties is required. From a biblical perspective, rape could legitimately be seen as rising to the same level of gravity and requiring the same penalty.
As a note, the American justice system is set up on the idea of life, liberty, and the pursuit of happiness. A minor offense is punished by a restriction of one's pursuit of happiness--some of their money is forfeited. A graver offense is punished by a restriction of one's liberty--they spend time behind bars, lose their driver's license, or something like that. The gravest offenses may be punished with the taking of the criminal's life.
So, there are inalienable rights with which each individual is endowed by God. However, a person may forfeit those rights to varying degrees.
Therefore, a Christian can justifiably be in favor of the death penalty because life is sacred.