No one wants to see babies killed.
Benevolent Persian rule is not the same as the Persian war machine. The Persians were flaunting global mastery. At war, they were as brutal as anyone else.
Actually, the Persians were noted for their benevolence towards conquered people, it's why their empire was so successful.
As far as the Hebrews not wanting to kill babies, maybe you should ask the Amalekites. God demanded they be killed to the last man, woman and child. In fact, when Saul was insufficient in his genocide (He was fine with killing babies, but man, he drew the line at Cattle the Hebrews could actually use!) God sent Samuel to curse him. This is all covered in 1 Samuel 15.
7 Then Saul attacked the Amalekites all the way from Havilah to Shur, near the eastern border of Egypt. 8 He took Agag king of the Amalekites alive, and all his people he totally destroyed with the sword. 9 But Saul and the army spared Agag and the best of the sheep and cattle, the fat calves[b] and lambs—everything that was good. These they were unwilling to destroy completely, but everything that was despised and weak they totally destroyed.
10 Then the word of the Lord came to Samuel: 11 “I regret that I have made Saul king, because he has turned away from me and has not carried out my instructions.” Samuel was angry, and he cried out to the Lord all that night.
You do not see the mourning in the song over the behavior of mankind? The Jews began by mourning their own evil, noting that evil brought greater evil upon them, and that this evil would continue to grow so that other conquerors would be happy as they committed their own evil--that of dashing Babylon babies against rocks.
Nope, what I see are a bunch of religious prudes wanting awful things to happen to other people because of imagined slights.
I noticed you carefully distinguished killing babies outside the womb and the killing of babies inside the womb. Imagine a future dirge of our time, "And they happily killed life inside the womb." It appears that in Biblical times as well as our own, can't have a baby standing in the way of what we want.
Fetuses aren't babies. Now, anti-abortion attitudes were drilled into me, and it was one of the last things I abandoned even after becoming an atheist. All that fetus porn I was exposed to in Catholic School had an effect. (Fetus Porn is when they go dumpster diving into the contents of medical waste containers and try to find something that kind of looks like a baby if you photograph it the right way).
Then I realized two realities.
1) A woman who doesn't want to be pregnant will find a way to not be pregnant.
2) The Right wing in this country has used abortion (among other issues) to get really stupid white people to vote against their own economic interests.
Rather, all the verses you never listened to on Sunday along with the homilies explaining the events of that day.
Oh, I promise you, they never talked about Jephthah, or Elijah's bears, or any of the other fun verses in the bible. Maybe my bible should have come with one of these.