Jepthah made a vow that whatever came out of his house first would be sacrificed to the LORD. (This is actually pretty strange; what other object in his house can get out by itself?)
Nevertheless, the vow was made and Jepthah's daughter was toast, literally.
Should he have done that? While not doing your vow is a sin, killing your own daughter seems to be an even more serious issue.
Excellent question.
Back in an agricultural society, people had animals around. So he was thinking it would be a cow, sheep, or goat. Nonetheless, the Talmud said he should not have taken that oath -- what if it was a horse or donkey? (Which can't be used as a sacrifice.)
Some Christians took this story as a message "oh, always fulfill your oaths." The traditional Jewish interpretation is disgusted by that possibility! If I swear to commit murder, or eat a cheeseburger, the oath is null and void because I can't override the word of God!
So Jephthah messed up in taking the oath. (He could have pledged "the first unblemished cow, sheep, or goat that would exit the yard.") But the far greater mistake was in carrying it out.
Jewish tradition has it that if someone made an oath with faulty information, and regrets ever having made it to begin with, he can have it annulled by going to a panel of three knowledgeable Jewish men, or one great rabbi. Our understanding is that Jephthah said "hey I'm the big military leader, the rabbi should be knocking on my door to annul my oath!" Meanwhile the rabbi said, "I'm the spiritual leader, that ignoramus should be knocking on my door, then I'll annul his oath!" (Recall that Jephthah had grown up as an outcast, and was finally the big man on top of the pyramid. For him to go ask someone for help would mean not being it anymore, which he couldn't face.) So the stubborn men each refused to swallow their pride and give an inch, and something terrible happened as a result. (You'll see all sorts of problems going on in the book of Judges. It was written by Samuel, who is framing the past few centuries of history as an explanation of why the people now needed a monarch.) Reminds me a lot of Dr. Seuss' "The Zax."
While the simple reading of the verse is that he did actually kill her, some commentaries suggest instead that she was forced to live out her life as a hermit, effectively rendering her "good as dead" from the perspective of human society. (Hey we're social animals.) But one way or the other, the story is intended as tragedy -- just because a Judge was a good national leader, he could still be a terrible person.
On a peshat level, Yiftach (see Shofetim 11) was not exactly the most educated or refined of individuals.
1) He was the son of a harlot (pasuk 1)
2) He was a gang leader of a group of no-goodniks (pasuk 3) who was only brought back and promoted because a tough guy was needed (pasuk 8).
3) He is not theologically sophisticated, and in speaking to the king of Ammon, asserts that their idol, Kemosh, exists as a divine power (pasuk 24)
Therefore, if we indeed say that he fulfilled his vow by sacrificing his daughter (which contrary to some comments, is the simple peshat in the pesukim - bewailing her virginity is bewailing that she won't live to marry and have children), then indeed, he erred and did a great sin in killing his daughter. But that is what we would expect of a theologically unsophisticated tough-guy leader.
As to what else could have come out to greet him, the idea was that someone might be driving sheep or cattle from the barn, and the first thing he would meet would be those sheep or cattle.
We don't know if he did actually sacrifice his daughter by killing her, but he should have sought the council of the Kohen Gadol (some say Pinchas was still alive).
There is, however, a formula set out in the last chapter of Vayikra:
Last chapter of book of Vayikra (Leviticus) first 5 verses:
copied from chabad.org translation (didn't paste that well)
1 And the Lord spoke to Moses, saying, אוַיְדַבֵּר יְהֹוָה אֶל משֶׁה לֵּאמֹר: 2Speak to the children of Israel and say to them: When a man expresses a vow, [pledging the] value of lives to the Lord, בדַּבֵּר אֶל בְּנֵי יִשְׂרָאֵל וְאָמַרְתָּ אֲלֵהֶם אִישׁ כִּי יַפְלִא נֶדֶר בְּעֶרְכְּךָ נְפָשֹׁת לַיהֹוָה: 3the [fixed] value of a male shall be as follows: From twenty years old until sixty years old, the value is fifty silver shekels, according to the holy shekel; גוְהָיָה עֶרְכְּךָ הַזָּכָר מִבֶּן עֶשְׂרִים שָׁנָה וְעַד בֶּן שִׁשִּׁים שָׁנָה וְהָיָה עֶרְכְּךָ חֲמִשִּׁים שֶׁקֶל כֶּסֶף בְּשֶׁקֶל הַקֹּדֶשׁ: 4And if she is a female, the value is thirty shekels; דוְאִם נְקֵבָה הִוא וְהָיָה עֶרְכְּךָ שְׁלשִׁים שָׁקֶל: 5And if [the person is] from five years old until twenty years old, the value of a male shall be twenty shekels, while that of a female shall be ten shekels; הוְאִם מִבֶּן חָמֵשׁ שָׁנִים וְעַד בֶּן עֶשְׂרִים שָׁנָה וְהָיָה עֶרְכְּךָ הַזָּכָר עֶשְׂרִים שְׁקָלִים וְלַנְּקֵבָה עֲשֶׂרֶת שְׁקָלִים:
Assuming his daughter was between the ages of 5 and 60, he would appear to "owe" either 10 shekels (if she's under 20) or 30 shekels (if she's over 20).
Someone perhaps explain why no commentators as yet appear to have picked up on this. (Note: I was intended to ask this as a question but noticed this had already been asked and felt posting this as an answer may have been the best way to handle it).
