Paul says this because there is more glory in salvation than there is in death. However justly a judge may condemn some-one, he earns more honor if he shows mercy, so that the guilty party is given an opportunity to mend his ways.
Also, he interprets more clearly the meaning of the words, "The letter killeth," declaring it to be that which we have said above, namely, that the Law showed sin, not caused it.
"Much rather doth the ministration of righteousness exceed in glory." For those Tables indeed showed the sinners and punished them, but this not only did not punish the sinners, but even made them righteous: for this did Baptism confer.
Again in another way he presents the same thought. Explaining his own words, "the letter kills," he calls the law "the ministry of condemnation," as the punisher of sins, not their cause. The Gospel, however, he calls "the ministry of righteousness," because it not only frees from punishment, but also makes sinners righteous. Therefore the Gospel will possess far greater glory.
[AD 384] Ambrosiaster on 2 Corinthians 3:9