Well, He tells us in his _Treatise on the Love of God_[1] that: "The
angelic nature could only commit sin from positive malice, without
temptation or motive to excuse, even partially. Nevertheless, the far
greater part of the Angels remained constant in the service of their
Saviour. Therefore God, who had so amply glorified His mercy in the work
of the creation of the Angels, would also magnify His justice; and in His
righteous indignation resolved for ever to abandon that accursed band of
traitors, who in their rebellion had so villainously abandoned Him."

On man, however, He took pity for several reasons. First, because the
tempter by his cunning had deceived our first father, Adam; secondly,
because the spirit of man is encompassed by flesh and consequently by
infirmity; thirdly, because his spirit, enclosed as it is in an earthly
body, is frail as the vessel which enshrines it, easily overbalanced by
every breath of wind, and unable to right itself again; fourthly, because
the temptation in the Garden of Eden was great and over-mastering; fifthly,
because He had compassion on the posterity of Adam, which otherwise would
have perished with him; but the sixth, and principal cause was this:
Almighty God having resolved to take on Himself our human nature in order
to unite it to the Divine Person of the Word, He willed to favour very
specially this nature for the sake of that hypostatic union, which was
to be the masterpiece of all the communications of Almighty God to His
creatures.

Do not, however, imagine that God so willed to magnify His mercy in the
redemption of man that He forgot the claims of His justice. No, truly; for
no severity can equal that which He displayed in the sufferings of His Son,
on whose sacred Head having laid the iniquities of us all, He poured out a
vengeance commensurate with His Divine wrath.