A man after the devil's own heart!
(George Lawson, "A Practical Exposition of the Book of Proverbs" 1821)
"A malicious man disguises himself with his lips, but in his heart he harbors deceit! Though his speech is charming, do not believe him, for seven abominations fill his heart!" Proverbs 26:24-25
An angry man is dangerous — but, if you are on your guard, the danger will soon be over.
The malicious man is far worse, and much more dangerous, for his hatred ferments in his heart, and in the mean time, he is projecting methods for wreaking his malevolence in such a manner as will be safest to himself, and most hurtful to its object. He is not like the dog that barks before it bites — otherwise you might stand to your own defense. But he is a dog that fawns upon you, and, when you are never dreaming of it, viciously attacks you, and inflicts an unexpected and dangerous wound.
Solomon warns you that your safety lies in refusing to trust him, even when he makes the largest professions of friendship. When he speaks charming words, believe him not, although he should swear to the truth of all he says. If you have any reason, from your knowledge of a man's character, or from his former behavior, to think that he is one of this stamp, and capable of such wicked conduct — his ardent professions of love should rather confirm than remove your suspicions of him. For the darkest designs are always covered under the greatest shows of virtue and friendship.
You may as safely believe the devil himself, as one who joins malignity of heart with flattery and caresses — for he is a man after the devil's own heart! His character is a compound of all those vices of the blackest and the vilest kind, which make a consummate villain, and render a man a disgrace to human nature, by his exact resemblance to those infernal fiends who are to be dreaded equally for their malice and subtlety.