It's not "preposterous," it's entirely logical. The contention is that there is no god. Therefore, all the harm done in the name of "god" is done by humans without any outside influence, ie, "god."
Humanity is a closed system. We exist on this planet without any other sentient influence. Without any other sentient influence, all harms done by humans originate with humans. All "god" is is an excuse. All harms done in the name of "god" are human harms done by humans. Removing "god" from the equation doesn't mean less harms, it means different reasons.
"God" is not harm. "God" is a tool, like guns are tools. You're claiming that without guns humans wouldn't kill each other. 10,000 years of swords and arrows beg to differ.