You know, it doesn't need to be Rimuru. He could send Veldora to protect the Animal Kingdom. Have others board on his back to make sure he doesn't go overboard.
There's zero reason to just accept the innocent civilians die. Another of Rimuru's "not the good guy"-scenes, albeit it's more stupid, considering NOBODY there sees the obvious way to send help.
The series is presenting him in the way of "being the good guy" and that's hard to watch. Imagine watching, say, Braveheart, but the English sodiers are being presented as good guys, in a lighthearted way right after raping and murdering Mel Gibson's wife. It's infuriating.