Jephthah’s question is a rhetorical one. It may sound as though he wants an explanation, but it becomes obvious from the next verse that he was not really interested to hear their position or negotiate with them.1 He is no longer interested in talking things over, but in killing his opponents.
3 And when I saw that you would not save me, I took my life in my hand and crossed over against the Ammonites, and the LORD gave them into my hand. Why then have you come up to me this day to fight against me?”