[Text: Am I the only one who thinks Jephthah made his oath/To have an excuse to kill his daughter?]
Explanation for non-biblical folks: Judges 11 is the charming story of how Jephthah promised YHWH that if YHWH gave him victory in battle, he would sacrifice the first thing* to come out of his house on his return as a burnt offering to YHWH. Now, when Jephthah came home, his daughter was there to greet him, and Jephthah makes a great fuss about how he cannot take back his vow.
But why was his daughter there? To greet her returning father as was right and proper. As he knew she would. And clearly he did not tell her about his oath, or she would have sent a goat or a slave** out instead. He knew she would be the first out of the house, he knew she would have to die.
Jephthah: J'accuse
*Most translation read "whatever" or in Judges 11:31. The NRSV reads "whoever." The Hebrew word is yatsa which Strong's concordance translates as "to come out of"; no subject is supplied for yatsa. A brief concordance study (i.e. reading the first page of the concordance for yatsa) shows that the word was used to refer to both people and things.
**Presumably she'd only send out a slave if Jephthah's oath required specifically that a person be sacrificed to YHWH.