Does ethics follow ontology?

Ontology is the philosophy of existence. What we believe about the nature of the existence of our world and others determines how we treat them.

Where this comes in, is that on numerous occasions I've seen the atheists here assert that religion leads inevitably to tyranny and brutality. This is not a universal tendency in history, and there are stories they don't tell about it ameliorating it.

I intend to put the shoe on the other foot. The atheist asserts that there is nothing transcendental in us. There is no spirit, and we aren't materially different from what surrounds us. Ultimately, this makes us like bugs or blades of grass, nothing more.

They can't offer a moral explanation why killing a man is worse than killing a roach, but they can offer claims they prefer it the other way. Preference changes quickly.

Is there any way a regime that states that it is atheist can avoid the tyranny and brutality that ALL such regimes in the past? I don't believe they have a way to avoid it.