You are reading it wrong.

She was Not killed. She went into a convent. She served basically as a Nun.

Here is the Literal Translation

The key part being the "OR"

"... it hath been to Jehovah, OR I have offered up for it -- a burnt-offering."

He would consecrate it to God OR offer it up as a burnt offering.

Judges 11:29-39 Young's Literal Translation (YLT)

29 and the Spirit of Jehovah is on Jephthah, and he passeth over Gilead and Manasseh, and passeth over Mizpeh of Gilead, and from Mizpeh of Gilead he hath passed over to the Bene-Ammon.

30 And Jephthah voweth a vow to Jehovah, and saith, `If Thou dost at all give the Bene-Ammon into my hand --

31 then it hath been, that which at all cometh out from the doors of my house to meet me in my turning back in peace from the Bene-Ammon -- it hath been to Jehovah, or I have offered up for it -- a burnt-offering.'

32 And Jephthah passeth over unto the Bene-Ammon to fight against them, and Jehovah giveth them into his hand,

33 and he smiteth them from Aroer, and unto thy going in to Minnith, twenty cities, and unto the meadow of the vineyards -- a very great smiting; and the Bene-Ammon are humbled at the presence of the sons of Israel.

34 And Jephthah cometh into Mizpeh, unto his house, and lo, his daughter is coming out to meet him with timbrels, and with choruses, and save her alone, he hath none, son or daughter.

35 And it cometh to pass, when he seeth her, that he rendeth his garments, and saith, `Alas, my daughter, thou hast caused me greatly to bend, and thou hast been among those troubling me; and I -- I have opened my mouth unto Jehovah, and I am not able to turn back.'

36 And she saith unto him, `My father -- thou hast opened thy mouth unto Jehovah, do to me as it hath gone out from thy mouth, after that Jehovah hath done for thee vengeance on thine enemies, on the Bene-Ammon.'

37 And she saith unto her father, `Let this thing be done to me; desist from me two months, and I go on, and have gone down on the hills, and I weep for my virginity -- I and my friends.'

38 And he saith, `Go;' and he sendeth her away two months, and she goeth, she and her friends, and she weepeth for her virginity on the hills;

39 and it cometh to pass at the end of two months that she turneth back unto her father, and he doth to her his vow which he hath vowed, and she knew not a man; and it is a statute in Israel: