I know it always supports big business. And it’s a reactionary force that people in power turn to when things go badly for the workers and they think of revolt.
But fascism also demands genocide and killing of millions of people and that would only hurt the capitalists, no? Because these are people who would otherwise be working and increasing profit used to increase productivity etc etc etc. But if you kill them you reduce the labor force and thus drive up the price of labor, reducing your profits, and thus what you can spend on improving productivity? And other countries which didn’t do so would become more advanced, leaving you behind.
To answer your question you should try another question.
What did fascism become when it was not defeated? Did it become something new and clearly identifiable as different to capitalism? Or did it simply morph back into liberalism again?
In Germany and Italy fascism was defeated with extreme violence but we can't say the same for Spain, Chile and others that reared their heads from time to time.
They morphed back into liberalism over time, why? Because liberalism is a more efficient machine for extracting profit. It lies to the proletariat and convinces them to get on with the work without fuss. Many proles will in fact do this, feeling that their lives are "good enough" to not rock the boat. This mode however is not an effective machine of mass violence, it cannot carry out a national mass-murder campaign against its opponents.
Fascism is brought out when the bourgeoisie feel threatened enough to require it as a tool of massive violence. Liberalism is brought back when the threat has passed. It does not oppose capitalism, it secures it.