Err - In the sense of wandering from the right way, the way of life.

Do they not err that devise evil?.... Certainly they do; they go astray from the right way, from the word of truth, from the Gospel of Christ, who contrive schemes to commit sin, and do mischief to their neighbours; or who "plough" (t) it, and sow it, and expect a fine harvest; but they will be mistaken, and find it will not turn to account, and that they have took a wrong course, and have gone out of the way: none more mischievous devisers or contrivers of evil than the Papists, and none more sadly and fatally err;
but mercy and truth shall be to them that devise good; who devise liberal things, to do good to the poor and needy; to their neighbours, their fellow creatures and fellow Christians: such receive grace and "mercy" at the hands of God, and his "truth" will appear in making good all promises to them; mercy and truth will preserve them from the evil way, and guide them in the right way, so that they shall not err as others do; neither from the doctrines of grace and truth, nor from the practice of them.
(t) "arant", Baynus; "arantibus", Amama; "verbum proprie significat arare", Piscator.

How wisely those consult their own interest, who not only do good, but devise it!

As usual, the interrogative negative strengthens the affirmative.
mercy and truth--that is, God's (Psalm 57:3; Psalm 61:7).

The proverb terminating (Proverbs 14:21) with אשׁריו (cf. Proverbs 16:20) is now followed by one not less singularly formed, commencing with הלא (cf. Proverbs 8:1).
Will they not go astray who devise evil,
And are not mercy and truth to those who devise good?
The part. חרשׁ signifies both the plougher and the artisan; but on this account to read with Hitzig both times חרשׁי, i.e., machinatores, is nothing less than advisable, since there is connected with this metaphorical חרשׁ, as we have shown at Proverbs 3:29, not only the idea of fabricating, but also that of ploughing. Just so little is there any reason for changing with Hitzig, against all old translators, יתעוּ into ירעוּ: will it not go ill with them...; the fut. יתעו (cf. Isaiah 63:17) is not to be touched; the perf. תעו (e.g., Psalm 58:4) would denote that those who contrive evil are in the way of error, the fut. on the contrary that they will fall into error (cf. Proverbs 12:26 with Job 12:24). But if הלא יתעו is the expression of the result which shall certainly come to such, then 22b stands as a contrast adapted thereto: and are not, on the contrary, mercy and truth those who contrive that which is good, i.e., (for that which befalls them, as Proverbs 13:18, cf. Proverbs 14:35, is made their attribute) are they not an object of mercy and truth, viz., on the part of God and of men, for the effort which proceeds from love and is directed to the showing forth of good is rewarded by this, that God and men are merciful to such and maintain truth to them, stand in truth to them; for חסד ואמת is to be understood here, as at Proverbs 3:3, neither of God nor of men exclusively, but of both together: the wicked who contrive evil lose themselves on the way to destruction, but grace and truth are the lot of those who aim at what is good, guarded and guided by which, they reach by a blessed way a glorious end.