@Billie Here are a few things to consider:
- if we vow to God to do something that is evil, we should not keep the vow. Imagine if I said, “God, I vow to lie if the sun rises tomorrow.” The sun rises tomorrow. What is the godly thing to do? Tell the truth of course! A vow is not a reason to violate God’s command!
- this story is in the book of Judges - the whole point of Judges is that ‘everyone did what was right in their own eyes’ - we are supposed to be horrified by much of what happens in this book. These people have lost their way.
- it is possible, though not necessarily the case, that Japheth did not kill his daughter, buy rather gave her in temple service to the Lord.
Christ grant you wisdom
Here is an article from the Gospel Coalition providing 6 reasons to consider the possibility that Jephthah did not sacrifice his daughter, but rather offered her in full time service to the Lord. I have included and excerpt below. The Lord grant you wisdom and peace in this matter.
The willing fulfillment of this vow by Jephthah’s daughter (11:36) appears to contradict the literal interpretation of a child sacrifice. Not only were such sacrifices clearly forbidden and abominated in Scripture (Deut. 12:31; 18:9–12; cf. 2 Kings 3:27; 23:10; Is. 57:5), but the concern of the text is never death, but always virginity. In 11:37, Jephthah’s daughter requests a two-month leave in order to lament her virginity. Then, in 11:38, the text records that while with her friends, she wept over the fact of her virginity. Then again, in 11:39, it is recorded that Jephthah fulfilled his vow to the LORD, and the text clearly describes how this vow was fulfilled—“that is, she did not know a man.” It appears, therefore, that Jephthah’s vow consisted of offering a member of his house to the full-time service of the LORD, and thus not to the normal duties of a household, such a marriage and having children. Service of this type in not unknown in the Old Testament (Ex. 38:8; 1 Sam. 2:22; cf.1 Sam. 1:11, 22–28).