en_tn_condensed/jdg/21/05.md

The people of Israel said, "Which of all the tribes of Israel did not come up in the assembly to Yahweh?"

The people are referring back to the assembly of the Israelites at Mizpah before they attacked the Benjamites.

For they had made an important oath concerning anyone who did not come up to Yahweh at Mizpah. They said, "He would certainly be put to death."

This is background information to explain to the reader the promise that the Israelites had made at Mizpah before they attacked the Benjamites. (See: rc://en/ta/man/jit/writing-background)

He would certainly be put to death

Here "He" refers to anyone who did not go to Mizpah. This can be stated in active form. Alternate translation: "We will certainly kill that person" (See: rc://en/ta/man/jit/figs-activepassive)