To add another truth to the mystery of Mary’s freedom from sin - we need to understand the perfection of freedom. God has/is the perfection of all virtues, all goodness. God possesses perfect freedom, and God cannot sin. It is not that He “chooses” not to sin: He cannot sin. There is
no darkness (no evil)
in Him at all (1 Jn 1:5). God cannot even be tempted to evil (James 1:13), because there is no desire for evil in Him, at all (see James 1:14).
And the Catechism teaches:
1733 The more one does what is good, the freer one becomes. There is no true freedom except in the service of what is good and just. The choice to disobey and do evil is an abuse of freedom and leads to “the slavery of sin.”<Cf. Rom 6:17>
We misunderstand “freedom” because it is very difficult to even
imagine a life without the weakness and brokenness we have in us due to original sin, and concupiscence. Our nature is deeply wounded - but Mary’s is not, and never was. Mary had
true freedom: thus her life was in the “the service of what is good and just.” She was full of
grace - and "Grace is a participation in the life of God. " (Catechism 1997). She had no desire for, nor did she have room for, in her heart anything evil. There was no part in her in “the slavery of sin.” Her life was
for “participation in the life of God.” Her freedom was directed to the perfection and the service of freedom: to desire, to live for, the good.