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.
"For if the ministration of condemnation is glory."
Also, he interprets more clearly the meaning of the words, "The letter kills," declaring it to be that which we have said above, namely, that the Law showed sin, not caused it.
"Much rather does 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.
[AD 384] Ambrosiaster on 2 Corinthians 3:9