According to the teachings of the Church, if one dies "in mortal sin without repenting and acceptin God's merciful love means remaining separated from him for ever by our free choice." (CCC 1033) Also, God does not predestine one to go to hell, for going to hell requires a willful and lasting individual choice to turn away from God to the very end. (CCC 1037).
Our call as Christians is to lead all souls to God through evangelization, including those on Death Row. We also know that God does not want "any to perish but to come to repentance" (CCC 1037). If we are not able to physically spread the Word to those waiting in a cell for their date with death, we can, through prayer, ask for God's merciful grace to pierce the hearts of the unrepentant that they may heed the call to conversion, to enter the "narrow gate." In fact, His very church, through the faithful, "implore the mercy of God" in our Eucharistic liturgy and daily prayers (CCC 1037).
Therefore, it is not only right that we pray for the conversion of souls and implore the mercy of God on those, who, sitting on Death Row, have turned their backs on the Father through their great sinful deeds, it is imperative that we do so.
If God does not want any soul to perish, then we shouldn't either.
O Heavenly Father,
You who desire the unification of all souls to you,
through your abounding mercy,
pour out your grace to those who, through their wicked deeds,
have turned against you and await final earthly punishment.
Enlighten their minds and their souls so that they come to love you
and make the fervent choice to turn to you through repentance.
Most especially, we ask you to have mercy on the "DC Sniper", John Mohammad,
who sits on Death Row and is scheduled for execution tonight.
May he desire to see you face to face
and heed your call to conversion before it is too late.
We ask this through Jesus Christ, our Lord.