10:17 — Whoever heeds instruction is on the path to life, but he who rejects reproof leads others astray.
The person who learns from his mistakes or heeds the instructions and corrections he receives can lead a life that pleases the Lord. The rejection of reproof will eventually enable the same person not only to go astray but will lead other people astray. “Apostates praise the wicked (28:10) and seek to make converts, perhaps to find security in an unenlightened consensus (cf. 1:10-19; 9:13-18).” (Waltke) The obstinate will not admit his mistakes but only make excuses; he is not on the path to life. His path will lead others astray and in turn, both he and his followers will fall into destruction.
10:18 — The one who conceals hatred has lying lips, and whoever utters slander is a fool.
Once again, this verse pertains to the tongue. It is about a fool who “spreads slander, concealing his hatred with lying lips.” (Waltke) That is, the fool slanders another person perhaps under the guise of simply conveying his concerns, observations, or supposed interest in the other person’s welfare by slandering him. He conceals his hatred all the while expressing his lying lips. “He who indulges so wicked and dangerous a passion, is a fool, whether he conceals it under the mask of friendship, or discovers it by reproaches and calumnies. It must neither by concealed nor published, but suppressed and extinguished.” (Lawson)
Longman takes it a little differently. He believes there should be some direct confrontation. “Proverbs understands that it is important to be open and honest with one’s words. If there is a legitimate gripe about something, the wise person will rebuke the other, with the purpose of helping and restoring relationship. Here, however, there is no intention other than to hurt, belittle, or demean the other person. Through such actions, relationships are destroyed.” (Longman)
10:19 — When words are many, transgression is not lacking, but whoever restrains his lips is prudent.
Because our hearts are not entirely pure our words will eventually catch up. The wise person carefully speaks and carefully avoids speaking too much. We are not wise enough to think through all our words; think more, speak less. Again, James 3:2 must be regarded. Not everything should be said and when one says anything, it should be said sparingly. “Indeed a talent for conversation is valueless both to the possessor and to the auditors, except it be connected with a talent for silence.” (Bridges) We must also remember that our tongues need the forgiveness of our Lord. His blood can wash away all our sins.
10:20-21 — The tongue of the righteous is choice silver; the heart of the wicked is of little worth. The lips of the righteous feed many, but fools die for lack of sense.
The wise or righteous are once against contrasted in their use of the tongue. What the wise have to say are “choice silver” and they “feed many.” On the other hand, the fool’s heart (which controls the tongue) is of little worth. Whereas the righteous man does good to many with his lips, the fool simply dies away on account of his folly. He brings ruin upon himself by his use of the tongue. Fools “have nothing inside [of their hearts] to share with others or even to sustain themselves.” (Longman)
The righteous man feeds many with knowledge, for he finds it sweet to himself, and wishes not to eat his morsel alone. His heart is a storehouse of provision for the soul, and like a hospitable landlord, he delights in distributing it to others. But the wicked die for want of heart. (Lawson)
 Whoever misleads the upright into an evil way will fall into his own pit, but the blameless will have a goodly inheritance.