Of course we should pray for him. The hardest of sinners in this life are the ones who require the most prayer.
" When Jesus heard this, he said to them, “Those who are well have no need of a physician, but those who are sick; I have come to call not the righteous but sinners.” Mk 2:17
“When we die, there will be three surprises in Heaven. First we shall find there people whom we never expected would be there; then we shall find people whom we were certain would be there, but are not. And third, by the mercy of God, I will be there!” -Fulton J. Sheen
May God pour out His Infinite Mercy upon Charles Manson and all of his victims.