Well, radical atheists' would blame Christians for the fall of American Indians and colonization worldwide.
I don't doubt for a moment that they would blame Christians.
But here are two aspects of that matter to keep in mind--
A] While Christian churches and individuals have indeed behaved oppressively towards non-believers, you are referring to centuries long past. There is hardly a Christian denomination anywhere that condones such actions today.
B] The wrongdoing of various Christians in previous centuries were overwhelmingly a matter of them taking advantage of the colonial urges of governments, not churches. The colonization of parts of the Americas and Africa, for example, gave an opening to a few churches to try to convert these "new" prospects to the Christian Faith in the wake of the territorial actions of those monarchies.
As awful as some of that was, it cannot be equated to extermination camps and the intentional genocide that secular dictatorships operating in our own lifetimes have employed.
What is the response? Also, I have heard Marxists claim capitalism kills millions via starvation and stuff worse than what Mao did, but I am skeptical.
That isn't factual. However, Marx postulated a certain historical theory that he said was inevitable and would have a good ending. It has never happened, but that's what makes a Communist be a Communist rather than, say, a Democratic Socialist.
To be more specific, industrial development would, he said, be so oppressive to the working classes that there would inevitably be a workers' revolution, which would in turn produce a dictatorship of the workers (!). That dictatorship would exterminate the old oppressors (how about that?), rule in their place, and then, ultimately, at some time in the future, fade away because the people would, thanks to the regimentation and re-education of the masses under that dictatorship of the workers, not be needed anymore. By then a new breed of people who shared the wealth voluntarily would be in place.
It all sounds great to some people, but 1) it never has shown any hint of moving beyond the dictatorship phase in any Marxist country of recent memory, and 2) there simply is no reason to think that the fading away part followed by a post-industrial paradise on Earth is anything by a talking point.