With the pure thou wilt show thyself pure; and with the froward thou wilt show thyself froward; rather, thou wilt show thyself adverse. The same root is not here used for the verb as for the adjective, as is done in the three preceding clauses.
The reason is well explained in the 'Speaker's Commentary:' "In dealing with the good, God shows his approval by manifesting attributes similar or identical in essence; in dealing with the wicked, he exhibits attributes which are correlative—in just proportion to their acts," but not identical.
God cannot "show himself froward"—he can only show himself opposed, antagonistic, an adversary. What the psalmist means to say is that, if men oppose and thwart God, he in return will oppose and thwart them.
But they will act in a perverse spirit, he in a spirit of justice and righteousness.