In some sense, the world has always been capitalist. But that might be more because of human nature. The psychology/sociology behind capitalism is the same as that of human nature (driven by survival, driven by social darwinism, driven by the ego -> which explains partially why communism failed, because we cannot erase human nature).
No. The world has not "always" been capitalist. Capitalism is a very specific system of market governance in which labor is voluntarily allocated and private property is protected by the state. Trade and mutually beneficial exchange are inherently human traits, but that is not the only component of capitalism.
Let's assume what you state here is correct. And if we reverse the equation. When the Sovjet government does something wrong (gulags and stuff like that), is it the fault of the state of is it the fault of communism?
"Communism" is a system that organizes both the state and the economy. "Capitalism" is a system that only organizes the economy. You can have a capitalist dictatorship or a capitalist democracy.
We can't, but it doesn't matter, we all know that climate change and environmental damage / loss causes lives. We can do estimates however, some probably already did. Air pollution is one of them, we're not talking about climate change, but about environmental damage. I consider that environmental damage. I also think environmental decline - even if it didn't affect humans but it does, but let's assume it doesn't affect humans - is still not desirable. Loss of ecosystems and biodiversity is not something i think that should be desirable. And we haven't talked about other negative effects environmental damage causes.
Well, on some level I agree-- air pollution and related illnesses can certainly be construed as deaths that result from "capitalism." However, I think the issue here is that deaths that result from free market activities are inherently more difficult to quantify; deaths that "result from" capitalism usually happen because of negligence that ripples through multiple causal factors, whereas deaths that "result from" communism are often due to direct government action.