When he speaketh fair, believe him not: for there are seven abominations in his heart.
Hateful people cannot be trusted, for they are also liars. Their kind words are only to hide their hate and to deceive you. Only fools believe them. These hateful people will slice you badly, as soon as it is helpful to their cause. Their hearts are full of wicked thoughts, even while they piously use gentle words and pretend to be friendly and virtuous.
There is a context for this proverb, an immediate one and a larger one. The immediate context describes hateful persons (Pr 26:24). These wicked murderers use words to pretend to be someone they are not. They plan and plot how to conceal their wicked malice, even while they are lying to you about their intentions. They will not get away with it for long, for they will be exposed before all good men (Pr 26:26).
The larger context is a collection of proverbs dealing with hateful talebearers and other deceivers (Pr 26:17-28). In twelve verses, Solomon listed the various ways in which hatred, lying, talebearing, and contentions lead to strife, trouble, and judgment. The world would be a better place if all such persons were thrown on the dunghill (Pr 26:23), but God will surely judge them in ways similar to what they planned for others (Pr 26:27).
Good words and fair speeches may deceive the simple and the sentimental, but prudent men reject mere words; wise men measure always and only by actions over time. Even a child’s character is known by his actions, whether he is pure and right (Pr 20:11). You can know a murderer by his fruits, not by his words (Matt 5:21-22; 7:16; Jas 3:12).
Seven is the perfect number in Scripture and means a complete amount. A hateful person has a full array of abominations in his or her heart, no matter what they say in public. And you will give them extra advantage to harm you, if you believe their lying words. Do not believe him or her; protect yourself by staying suspicious of such persons.
Cain was such a person. He talked to Abel, but a knife was in his hand, and hatred was in his heart (Gen 4:8). Joseph’s brethren comforted their father, though they were the very cause of his grief (Gen 37:35). Saul offered David his daughter, only for the purpose of killing him (I Sam 18:17). Absalom waited two whole years and begged for his brothers to come to a party, so that he might kill Amnon (II Sam 13:22-29). Judas promised undying loyalty to Jesus Christ, but betrayed him with a kiss that very night (Matt 26:35).
How can you know a person is hateful, since this proverb is about them (Pr 26:24)? Here are a few traits: they do not forgive quickly and thoroughly; they hold on to offences; they assume the worst about others; they allow anger to simmer beyond one day; they are self-centered; they can criticize cruelly; they get angry out of proportion for the offence; they get bitter; they can be sarcastic; they are not actively kind to those below them; they are proud; they treat enemies poorly; they are glad when enemies are hurt; etc.
What are the lessons? First, be vigilant around a hateful person. Reject his words, for he is a liar. His heart is full of wicked malice, but he covers it with false kindness. Second, be faithful in all your relationships, even in your heart, lest you even appear like this despicable person. Love even your enemies; forgive every offence quickly and completely. To the degree you show love and mercy, God will show the same to you.