Musar к Шмуэль Б 3:46

Do not believe the counsel of one who hates and is inimical, as it is said: "But the kisses of an enemy are importunate" (Prov. 27:6). And know that many people express love with their words but it is possible that they are total enemies in their heart, and it is necessary that you recognize them and do not put your faith in them as it is said: "He that hates dissembles with his lips while he lays up treachery within him" (Ibid. 26:24). And even though he speaks to you with a smooth tongue and speaks tender words, do not incline your heart to him as it is said: "When he speaks graciously, do not believe him for there are seven abominations in his heart" (Prov. 26:25). And this is the type of conduct that Joab showed to Abner, son of Ner (II Sam. 3:27) and to Amasa, son of Yeter (II Sam. 20:9-10) (peace and love) until he killed them, and as Ishmael son of Nethaniah did with Gedaliah, son of Ahikam until he slew him (Jer. 41:1-2). One of the Sages said: "The best plan of those you can plot against your enemy is to turn him to your love (to make a friend of him) if you can."