Elijah and the 450 Prophets

Someone recently asked me:

I’m confused by Elijah killing 450 prophets…that just seems so cruel and excessive. Like, if someone told me that worshipping/following God meant killing hundreds of people — or anyone — I’d be like “uhm I’m not sure about this God.” I understand that it was a very different culture and time but I just don’t get it. My culture Bible doesn’t explain it either… Any insights?