To have pagan gods objecting to slavery seems to be the pot calling the kettle black, as according to the Enuma Elish, they created humanity to serve them and their worshippers are all slaveholding societies. I have done a fair bit of thinking about the cherem warfare in the Hebrew Bible, putting cities to the sword. In Canaan and Assyria, at least, it does not seem to have been a war crime.

Here’s Mesha, king of Moab from 830 BCE > I proceeded by night and I fought with it from the crack of dawn to midday and I took it and I slew all of them, 7,000 men and boys and women and girls and maidens because I had put it under ḥerem (in Moabite: החרמתה) to Ashtar-Chemosh.

and Ashurbanipal, king of Assyria > The people of Sais, Piṭiṭi (and) Ṣi’nu and the rest of the cities that had joined them (and) plotted evil, young and old, they struck down with the sword. No one among them was spared.

Naturally, the gods of these peoples would be incensed by that because they are their worshippers, but as an act of war it wasn’t uncommon. I still wanted to voice the objection, though.

I was initially a little puzzled by your remark that Baal came to the defense of the God of the Judeans, he doesn’t. You are quite correct that in the Hebrew Bible, El, God of Israel and Baal are rival gods, but the El Baal is talking about there is the Canaanite El, whose son he is and who was sometimes equated with Kronos in Hellenized sources. I wanted to make it clear that they were, at least in a sense, different gods, by saying that Israel’s god has a resemblance to Baal’s father.

You are correct that El or Assyrian Ilu could refer to a major deity in general, that’s why I specified El, God of Israel, a title used in the Hebrew Bible at Genesis 33:18-20