I kill ... I wound ... I will make mine arrows drunk with blood, and my sword shall devour flesh. Deuteronomy 32:39-42
This is the sixth time that the same stupid story is repeated in Judges (See Jg 2:11, 3:7, 3:12, 4:1, 6:1 for the others), so you know by now what's going to happen.
The Israelites do evil in the sight of the Lord.
The children of Israel did evil again in the sight of the LORD, and served Baalim, and Ashtaroth, and the gods of Syria, and the gods of Zidon, and the gods of Moab, and the gods of the children of Ammon, and the gods of the Philistines, and forsook the LORD, and served not him. Judges 10:6
God gets angry and sells them as slaves.
The anger of the LORD was hot against Israel, and he sold them into the hands of the Philistines, and into the hands of the children of Ammon. 10:7
The Israelites cry out to God.
The children of Israel cried unto the LORD, saying, We have sinned against thee, both because we have forsaken our God, and also served Baalim. 10:10
God slaughters the people he sold the Israelites to (which is what this killing is about).
A few details change each time the story is told: the number of years that the Israelites are enslaved, the people that he sells them to, and the person that he chooses to help him with the massacre. This time God chooses Jephthah.
As usual, everything starts to go to hell when the spirit of the Lord comes upon Jephthah.
The spirit of the LORD came upon Jephthah, and he ... vowed a vow unto the LORD, and said, If thou shalt without fail deliver the children of Ammon into mine hands, Then it shall be, that whatsoever cometh forth of the doors of my house to meet me, when I return in peace from the children of Ammon, shall surely be the LORD's, and I will offer it up for a burnt offering. 11:29-31
Did you catch that? The spirit of the Lord comes upon Jephthah and he promises to kill whatever comes out to greet him if God will help him massacre the Ammonites. God not only approved of Jephthah's vow, he inspired it.
And, of course, God comes through with his end of the deal by giving Jephthah "a very great slaughter."
So Jephthah passed over unto the children of Ammon to fight against them; and the LORD delivered them into his hands. And he smote them ... even twenty cities ... with a very great slaughter. 11:32-33
God delivered 20 cities into Jephthah's hand and "he smote them ... with a very great slaughter." And then Jephthah came home.
(I gave each city the usual 1000 slaughter amount, for a total of 20,000.)