If you reduce fascism to "let's genocide everyone else", of course the ideology doesn't make sense. I would say the idea of fascism is more about doing what's best for the nation or people, guided by a strong centrally led state, and a belief that imperialism and war can be appropriate tools to further this goal. It's adjacent to ultranationalism (though it doesn't have to be about the nation).

For example to name the most well-known example, Hitler's goals were:

- unite the German people under one country

- more lebensraum (space to live) for said German people

- get rid of undesirables who hold the Germans back (Jews, disabled people, Gypsies, homosexuals, etc)

If you start from the axioms that the German people are the best and should be elevated and all other people don't deserve compassion, it's not hard to arrive at those goals by pure reason. Of course those are bad axioms from our point of view, but you can't reason yourself away from your axioms.