The law demands good works and uses its terror--rejection, shame, fear of punishment, unanswered prayer, personal tragedy, etc.--as motivation. Here performance is a necessity to secure the blessings and avoid the curses. Grace, on the other hand, allows us to serve on a different basis--not from fear but on the basis of love and gratitude, from appreciation and gladness for blessings freely given and freely received.
Grace is not against good works It simply does not bless on the basis of good works. We receive blessing from God based solely on the merits of His Son--blessings freely given to us in Christ and nowhere else. The completeness that is in Christ mean deliverance from trying to 'be good' and 'do right' in order to be accepted by God.