The only way to be 100% certain that person will never again commit a murder, rape, etc, is to remove them from the living. Very few dead people have killed again.
The Fifth Amendment specifically provides for the death penalty: No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.
Therefore, with due process, a person can be executed.
Personally, I don't like my tax dollars paying for the hundreds of thousands of people in prisons. IMHO, these criminals are treated far too kindly and should be forced to work to earn their keep. The Thirteenth Amendment specifically allows this: Section 1. Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.Thus, convicts can be used as slaves and or as involuntary servants(although, I don't really grasp the difference...)
In the late nineties, when both Texas and Florida were executing murderers as fast as the appellant process allowed, both states saw a decline in murders.
Were someone to "find God" on death row, that should never be a deterrent to carrying out their sentence. In the case of Christians, for example, after their execution their souls would be in Heaven, so they really have nothing to fear from death, not to mention the fact that the forgiveness of God is not the forgiveness of the state and the separation of church and state people should be the first to point that out, yet they never do...