Jamieson, Fausset and Brown on 2 Samuel 20:9

9. took Amasa by the beard with the right hand to kiss him--This act, common with two friends on meeting when one of them returns from a journey, indicates respect as well as kindliness, and the performance of it evinced the deep hypocrisy of Joab, who thereby put Amasa off his guard. No wonder, then, that while this act of friendly gratulation after long absence occupied Amasa's attention, he did not perceive the sword that was in Joab's left hand. The action of Joab was indeed a high compliment, but neither suspicious nor unusual and to this compliment, Amasa paying attention and no doubt returning it with suitable politeness, he could little expect the fatal event that Joab's perfidy produced.

2Sa 20:10-13. Amasa Is Slain.