Because he did what his flesh told him to do.
He did what his flesh told him to do in ch7:14-25 of romans
I really wish you could allow for the possibility you just could be wrong on this and study it hard, depending fully on the holy spirit for enlightenment.
Sin DID NOT through the law arouse all manner of concupiscence in Paul the christian and by the commandment slay him for he lived under grace not law as a christian. He died to a law of righteousness therefore the law COULD NOT condemn Paul the christian