Judges 11 1) The Spirit of the Lord had already come upon Jephthah before he made his vow; showing that God was already intending to give him victory. (v29) 2) Jephthah rashly made a vow to God of his own accord, which was never affirmed by God nor required by God. (v30-31)
Even on a surface level reading of the passage, it would be difficult to defend that it was ever God's desire that Jephthah sacrifice his daughter. And, if he knew the character of God as proclaimed by God himself in Exodus 34:6-7, he could have repented and received grace and he still would have had his daughter.
This is just modern people creating a new interpretation of old writings. Apparently Abraham also didn't know about the "grace", since he was decided to kill his son and didn't even tried to ask God otherwise. God also was not worried at all, since he didn't care to enlighten Jephthah about his "grace" (even for the sake of the child).
The point of the story seems to be that you shouldn't go round making stupid oaths, not that you should sacrifice your children.
That's the whole point, certainly this was viewed as tragic but normal. In other words, child sacrifice was not recommended but certainly was within realm of possibility for ancient jewish religion (and of course for all other religions around that area).