Augustine of Hippo

"Because they turned evils against You." Now this punishment shall be recompensed to them, because the evils which they supposed to hang over them by Your reign, they turned against You to Your death. "They imagined a device, which they were not able to establish" [Psalm 21:11]. They imagined a device, saying, "It is expedient that one die for all:" [John 11:50] which they were not able to establish, not knowing what they said.

Exposition on Psalm 21 ↗

Thomas Aquinas

"For." Here the guilt of sinners is set forth, and it is described in two respects. First, as to the endeavor of their work; and as to this he says here: "For they have turned," etc.: because although they cannot accomplish it, still they endeavor as much as they can to inflict evils. Ps. 34: "They repaid me evil." Or, "they have turned," because the evils threatening them they wish to inflict on others. …

Exposition on the Psalms of David ↗

Bonaventure

Another evil counselor is one who turns good things into evil. The Psalm says: 'For they turned evils against you; they devised counsels which they were unable to establish; for you shall make them turn their back.' It is evil when someone says something by which the counsel of Christ could be considered evil — as when someone condemns the counsel of entering holy religious life. It is foolish, dangerous, pernicious, and reckless to turn into evil that which Christ declared good.

Collationes de Septem Donis, Collation 7 ↗