en_tn/2sa/08/02.md

Then he defeated

Here "he" refers to David who represents his soldiers. Alternate translation: "Then they defeated" (See: rc://en/ta/man/translate/figs-synecdoche)

measured their men with a line ... He measured off two lines to put to death, and one full line to keep alive

Here the "line" is a "rope." David had the soldiers lie down on the ground to be measured and sorted into three groups. The men in two groups were killed, and in the third, they were allowed to live.