From the beginning of my ministry, I have preached and taught consistently against the death penalty (see www.priestsforlife.org/deathpenalty). I do not believe it should be used and have joined efforts to abolish it.
At the same time, there's a difference between capital punishment and abortion. Put simply, abortion can never be justified; capital punishment can sometimes be justified. Abortion is intrinsically evil, which means that no circumstances can ever make it right. Capital punishment, on the other hand, is evil when used in the wrong circumstances, but can sometimes be used in the right circumstances.
Capital punishment can never be carried out on an innocent person. That would defy its very definition. Abortion, on the other hand, is always carried out on an innocent person. Otherwise, that would defy its very definition.
In those rare circumstances where it has been justified, capital punishment was carried out precisely for the defense of life. Abortion, on the other hand, is carried out precisely for the destruction of life.
There is a substantial difference between a tiny child, growing in her natural environment, and a convicted criminal who poses a threat to the well-being of society. Yet more children are killed by abortion in America every five days than have ever been executed by capital punishment.
The Bible, and 2000 years of Catholic teaching, recognize the state's right and duty to protect its citizens, even by force. Romans 13:1-5 reads,
"Let every person be subject to the governing authorities. For there is no authority except from God, and those that exist have been instituted by God. …. For rulers are not a terror to good conduct, but to bad…If you do wrong, be afraid, for he does not bear the sword in vain; he is the servant of God to execute his wrath on the wrongdoer."
In his encyclical The Gospel of Life, Pope John Paul II makes a clear distinction between a practical "no" to the death penalty and an absolute "no" to abortion. In regard to the state punishing wrongdoers, he writes, "the nature and extent of the punishment must be carefully evaluated and decided upon, and ought not go to the extreme of executing the offender except in cases of absolute necessity: in other words, when it would not be possible otherwise to defend society. Today however, as a result of steady improvements in the organization of the penal system, such cases are very rare, if not practically non-existent" (56).
Then he goes on to say, in distinction, "If such great care must be taken to respect every life, even that of criminals and unjust aggressors, the commandment "You shall not kill" has absolute value when it refers to the innocent person." (57)
Simply put, "You shall not kill" applies even to the criminal, but with exceptions. "You shall not kill" applies absolutely to the innocent (born and unborn), without exceptions.
Can one still be pro-life and support the death penalty in certain instances? The answer is yes.