The "deceit" of "those who imagine evil" can work nothing but evil to those whom they advise. The "counselors of peace" have joy in themselves, and impart it to others also.

Deceit is in the heart of them that imagine evil,.... That secretly devise mischief in their hearts against their neighbours; that plough evil, as the word (g) signifies, and sow discord among men; and by many artful and deceitful methods promote contention and division, in order to answer some base designs of theirs; but sooner or later they are deceived themselves, are disappointed of their views, the consequence of which is vexation and sorrow;
but to the counsellors of peace is joy: such who consult the good of others, who advise to peace, concord, and unity; who seek to cultivate it in their families and neighbourhoods, and in the church of God, in which, if they succeed, they have joy and pleasure; if not, they have a satisfaction, in their own minds and consciences that they have done what is right and good; such have a conscience peace now, and an eternal one hereafter; or, as Aben Ezra calls it, the joy of salvation; see Matthew 5:9.
(g)

Deceit and falsehood bring terrors and perplexities. But those who consult the peace and happiness of others have joy in their own minds.

that imagine--or, "plan" (Proverbs 3:29). They design a deceitful course, to which, with all its evils and dangers to others and themselves, the happiness of peace-makers is opposed (compare Matthew 5:9; Romans 12:18).

20 Deceit is in the heart of him who deviseth evil,
But those who devise peace cause joy.
Regarding the figure of forging, fabricating (lxx, Aquila, Symmachus, and Theodotion, τεκταίνειν), or of ploughing, which underlies the phrase חרשׁ רע, moliri malum, vid., at Proverbs 3:29. That deceit is in the heart of him who deviseth evil (בּלב־חרשׁי רע, as is correctly punctuated e.g., by Norzi) appears to be a platitude, for the חרשׁ רע is as such directed against a neighbour. But in the first place, 20a in itself says that the evil which a man hatches against another always issues in a fraudulent, malicious deception of the same; and in the second place, it says, when taken in connection with 20b, where שׂמחה is the parallel word to מרמה, that with the deception he always at the same time prepares for him sorrow. The contrast to חרשׁי רע is יועצי שׁלום si ח, and thus denotes not those who give counsel to contending parties to conclude peace, but such as devise peace, viz., in reference to the neighbour, for יעץ means not merely to impart counsel, but also mentally to devise, to resolve upon, to decree, 2-Chronicles 25:16; Isaiah 32:7.; cf. יעץ על, Jeremiah 49:30. Hitzig and Zckler give to שׁלום the general idea of welfare (that which is salutary), and interpret the שׂמחה as the inner joy of the good conscience. Certainly שלום (R. של, extrahere, in the sense of deliverance from trouble) means not only peace as to the external relationship of men with each other, but also both internal and external welfare. Thus it is here meant of external welfare; Hitzig rightly compares Jeremiah 29:11 with Nahum 1:11 to the contrast between שׁלום and רע. But as מרמה is not self-deception, but the deception of another, so also שׂמחה is not the joy of those who devise the device in their hearts for the deception of others, but the joy they procure for others. Thoughts of peace for one's neighbour are always thoughts of procuring joy for him, as thoughts of evil are thoughts of deceit, and thus of procuring sorrow for him. Thus וליועצי is an abbreviated expression for ובלב יועצי.

Deceit - They whose hearts devise mischief shall be deceived in their hopes, and bring trouble upon themselves: but they who by good counsels labour to promote peace, shall reap the comfort of it themselves.