Would you like that, in another way also, I should make what I say plainer? Let us look into their case, [to those] who avenge themselves even justly. For concerning the wrongdoers, that they are the most worthless of all people, warring against their own soul, is surely plain to every one.But who avenged himself justly yet kindled innumerable ills and pierced himself through with many calamities and sorrows? The captain of David’s host. For Joab both stirred up a grievous war and suffered unnumbered evils; not one of them would have happened had he but known how to exercise self-control. Let us flee therefore from this sin and neither in words nor deeds do our neighbors wrong.
So with the wicked severally, because they have evil not upon the tongue, but under the tongue, in the words of their mouth they hold out sweet things, and in the thoughts of their heart are plotting mischiefs. For it is hence that Joab held the beard of Amasa with his right hand, whilst secretly putting his left hand to his sword, he shed out his bowels. For to hold the chin with the right hand is to caress as if in kindness. But he puts his left hand to his sword, who in secret strikes in malice. Hence too it is written concerning their head himself; "Under his tongue is mischief and pain." For he that doth not display openly the ill that he designs, does not put forth on the tongue the mischief and pain of those, whose destruction he aims at, but keeps them close under the tongue.
[AD 407] John Chrysostom on 2 Samuel 20:9-10