If man is a good, and cannot act rightly unless he wills to do so, then he must have free will, without which he cannot act rightly. We must not believe that God gave us free will so that we might sin, just because sin is committed through free will. It is sufficient for our question, why free will should have been given to man, to know that without it man cannot live rightly. That it was given for this reason can be understood from the following: if anyone uses free will for sinning, he incurs divine punishment. This would be unjust if free will had been given not only that man might live rightly, but also that he might sin. For how could a man justly incur punishment who used free will to do the thing for which it was given? When God punishes a sinner, does He not seem to say, “Why have you not used free will for the purpose for which I gave it to you, to act rightly”? Then too, if man did not have free choice of will, how could there exist the good according to which it is just to condemn evildoers and reward those who act rightly? What was not done by will would be neither evildoing nor right action. Both punishment and reward would be unjust if man did not have free will. Moreover, there must needs be justice both in punishment and in reward, since justice is one of the goods that are from God. Therefore, God must needs have given free will to man. [On Free Choice of the Will, II.I, p. 36; emphasis added]
If God gave us free will with the intention that we should be free to use it to sin, then it would be unjust for him to punish us if we sin, says St. Augustine: for we would only be putting it to one of the uses for which God gave it to us. But this is wrong; God did not give us free will for that purpose. He gave us free will in order that we might freely do that which is good. Consequently when we sin, we abuse the gift that he has given to us, and thereby become subject to just punishment.
Note also that he insists upon what we have seen before. That is, justice in punishing us for sin demands that we have free will: “What was not done by will would be neither evildoing nor right action. Both punishment and reward would be unjust if man did not have free will.” If we lack free will, and if our sins are compelled in some way, then they are not actually sins, properly speaking; it would therefore be unjust to punish them as though we were actually responsible for them. This is why Catholic moral teaching insists that compulsion removes guilt, either partly or completely (depending upon the compulsion).
Lastly, note again that St. Augustine insists upon the reward that is justly due to those who do good. But this would only be just if there is a sense in which our good works may be truly said to be our own, and this cannot be said if we do not have free will. St. Augustine was Catholic; he firmly believed that our good deeds merit a reward from God (although, of course, they are completely inadequate as a means by which we may receive initial justification; we may only receive that by means of God’s grace alone, as we have seen many times).