How exactly would he otherwise have a chance to resurrect his people? Even by becoming a demon lord he's only promised less than 5%, if memory serves. He's not killing those people just for the laughs. Not even to make an example out of them (although that's a much needed side effect in that world). He's harvesting their souls to undo the damage they caused by attacking.
I said it before, and I'll keep saying it: None of those Falmuth folks would have died if they hadn't attacked first. Did they try to negotiate with Jura Tempest about the trade issues? No. They just thought they would attack, slaughter, enslave and loot & pillage the place and be done with it. It's insane to blame Rimuru or his country for anything. Trade is supposed to be a competition, anyway. Every country here on Earth is struggling with the challenges or benefitting from the fruits of international trade as well. That's just how it goes.
You aren't actually a spy from Falmuth, are you, KrayZ33? Or maybe you are working for Clayman.