Ten young men: that is, ten soldiers. These soldiers are further described as armor-bearers. Their usual job was to carry armor for Joab and fight alongside him during the battle. They were something like “bodyguards” (Contemporary English Version). Good News Translation‘s “soldiers” is not a good model here, because it is not as specific as the Hebrew.

Killed him: that is, they finished the task already started by Joab himself. This is reworded in New American Bible to say “killed him with further blows.” Others may prefer to say “finished the job of killing him.”

Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on the First and Second Books of Samuel, Volume 2. (UBS Helps for Translators). New York: UBS, 2001. For this and other handbooks for translators see here .