God loves killing children, but he wants you to believe that other gods love it more. In doing this, he is attempting to assert himself as the less blood-thirsty of your options. But, I’m not buying it. Here’s why: In the midst of all this child killing going on throughout the bible, god says this:
” When the Lord your God cuts off from before you the nations which you go to dispossess, and you displace them and dwell in their land, take heed to yourself that you are not ensnared to follow them, after they are destroyed from before you, and that you do not inquire after their gods, saying, ‘ How did these nations serve their gods? I also will do likewise.’ You shall not worship the Lord your God that way; for every abomination to the Lord which He hates they have done to their gods; for they burn even their sons and daughters in the fire to their gods. Whatever I command you, be careful to observe it; you shall not add to it nor take away from it.” Deuteronomy 12:29-32
He is using the example of burning children to convince people that other religions/gods are bad; a fear tactic, if you will. And yet, god himself is guilty of this very action. He has allowed it and even welcomes it as a tribute and offering. I give you the story of Jephthah:
” And Jephthah made a vow to the Lord, and said, ‘If you indeed will deliver the people of Ammon into my hands, then it will be that whatever comes out of the doors of my house to meet me, when I return in peace from the people of Ammon, shall surely be the Lord’s and I will offer it up as a burnt offering.” Judges 11:30-31
God holds up his end of the bargain, so…
” When Jephthah came to his house at Mizpah, there was his daughter, coming out to meet him with timbrels and dancing; and she was his only child. Besides her he had neither son nor daughter. And it came to pass, when he saw her, that he tore his clothes, and said, ‘Alas, my daughter! You have brought me very low! You are among those who trouble me! For I have given my word to the Lord, and I cannot go back on it.’ So she said to him, ‘My father, if you have given your word to the Lord, do to me according to what has gone out of your mouth, because the Lord has avenged you of your enemies, the people of Ammon.’ Then she sad to her father, ‘Let this thing be done for me: let me alone for two months, that I may go and wander on the mountains and bewail my virginity, my friends and I.’ So he said, ‘Go.’ And he sent her away for two months; and she went with her friends, and bewailed her virginity on the mountains. And it was so at the end of two months that she returned to her father, and he carried out his vow with her which he had vowed. She knew no man.” Judges 11:34-39
So… how is this offering of a burnt child any different from the offerings to the other gods he warned his followers about? Oh, that’s right- it’s not.
Common Sense 
I think there’s a bit more to the passage in Judges. When you consider the entire narrative many scholars have noticed a “Judges Cycle” where 1. Israel falls into sin/oppression (which is not simply “doing bad things”), 2. God raises up a judge/leader, 3. Israel is restored to peace/delivered, and 4. repeat the cycle. I know you’re an atheist but bear with me (especially when I start talking about the role of God in the story, but play along… we’re playing the narrative criticism game! ha! 🙂 ).
I think that it’s noteworthy that this structure can also be viewed as a downward cycle or a downward-spiral in which the phases somewhat get muddled and the circumstances become worse. It could be argued that God actually gradually leaves the picture. I think this is evident in the text- the first judge is so good that there’s only a couple verses on what he did! As the cycle continues things become more complicated and dire, ultimately ending in some of the most darkest of texts in the Old Testament (re: Judges 19). The judges become worse and worse. If I remember correctly, I do not think there is any textual interaction with God towards the end of the book in Judges 19.
Notice that there is no speech from God in the Japhthat text you referenced. For all we know, it could be Japhthah who is behaving rash and unrighteous- maybe he won the battle on his own and claimed that God gave it to him (gee, how many times has that happen in human history?). In the text there is no command from God to sacrifice, make a vow, etc. I would place Jephthah somewhere on this downward spiral. An OT professor I know once told me that a BCE-Hebrew reading or hearing of this text would arouse a lot of discomfort (and rightly so… I mean the guy freakin’ decides to kill his daughter…). He would argue that people who heard this story would actually regard Japhthah as a fool.
Just some thoughts…
I will not disagree that with the notion that there is a “Judges Cycle”, however, it merely strengthens my argument that god was in fact OK with Japhthah burning his child as offering. The lord/god, has a hand in all of the actions and events taking place throughout this story. In fact, this entire episode starts off with the spirit of the lord entering into Jephthah (“Then the spirit of the Lord came upon Japhthah, and he passed through Gilead… Judges 11:29). And, after Jephthah makes his vow to the lord, it specifically says, “…And the Lord delivered them into his hands.” Judges 11:32. Meaning, god held up his end of the bargain and now Japhthah must do the same. There was no plea from Japhthah to god asking for the life of his only daughter to be spared and there was no intervention from god to prevent the act. There is clear evidence throughout the book of Judges that god did in fact intervene as he saw fit. An example being in the following chapter about the birth of Sampson: God sends down an angel to impregnate a “barren” woman with a child who will become the savior of Israel, the husband was not around for the conversation and begs the lord to send the angel back so they could ask him/her/it to elaborate on just how they were supposed to care for this child. Which he did. I just think, given the context of Judges as a whole, that there should have been some sort of intervention if he (god) was in fact opposed to the burnt offering of children. I appreciate the thoughts!
Some more baby killing… and animals too: “Now go, attack the Amalekites and totally destroy all that belongs to them. Do not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys.” 1 Samuel (15:3)
So gnarly!
Heartwarming, isn’t it? 😉