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]])
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]])