True.

But there does not seem to be an equivalent page for 'Mass killings in capitalist regimes'

Even if it is documented, does it not show that how it is organised currently has an imbalance, likely from Anti-communist propaganda, things like red scare etc?

So, cannoted be unbiased, right? My focus was on that. There is no unbiased source, but only ones which acknowledge biases and maybe try to ensure that the biases do not affect the quality of the source too much.

Might I recommend King Leopold's Ghost if you want a straight up history book of the Belgian Congo. No shade against Heart of Darkness, I think that's a worthwhile read; but reading KLG first will give you more context to appreciate HoD. Or maybe read it after, idk, up to you