Proverbs 10:18 He that hideth hatred with lying lips, and he that uttereth a slander, is a fool.
Proverbs 10:18 NRSV Lying lips conceal hatred, and whoever utters slander is a fool.
Proverbs 10:18 NIV He who conceals his hatred has lying lips, and whoever spreads slander is a fool.
The first part describes hatred kept secret with fair words.
More light is shown on the first part by a similar proverb –
Proverbs 26:24 He that hateth dissembleth with his lips, and layeth up deceit within him; 25 When he speaketh fair, believe him not: for there are seven abominations in his heart. 26 Whose hatred is covered by deceit, his wickedness shall be shewed before the whole congregation.
An example is seen when Joab feigns a warm welcome to Amasa with murder in his heart.
2 Samuel 20:9 And Joab said to Amasa, Art thou in health, my brother? And Joab took Amasa by the beard with the right hand to kiss him. 10 But Amasa took no heed to the sword that was in Joab’s hand: so he smote him therewith in the fifth rib, and shed out his bowels to the ground, and struck him not again; and he died…
This behaviour is foolish because God will judge the perpetrator.
The second part is straight forward.
It is foolish to slander others because that too will result in judgment.
The commandment is to love our neighbour as ourselves.
If we love them, we will not hate them or lie to them or slander them.
In Christ, this love must be extended to our enemies.
Matthew 5:44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
I hope this helps.