How is Heart of Darkness a comment, condemnation, of the vilest scramble for loot, ivory, in the history of exploration?

I would say that using the word "comment" is too weak to
describe the theme of this novel. It is definitely a "condemnation" of colonialism.
Conrad believed that greed was behind colonialism. Why else would one country use its
might to subdue another? It has to be about the money - loot, ivory, gold - other things
that have value on the world market. The colonizing nation often veils its greed with
lofty phrases such as duty, responsibility, converting the masses, improving native
life, but as we see in this novel, often what happens is exploitation and worse, cruelty
and inhumanity to our fellow man.

Historically, the purpose
of exploration was to find gold, or trade routes (which would result in more gold for a
nation), or more national resources, or more living space. Sometimes nations explored
for purposes of evangelizing nations. No matter what the original purpose, however,
Conrad shows that if a nation shirks its duty towards cultures that are less developed
than they, the results are destructive and, in this novel, evil, for those colonized
countries. Why? Because, man's heart is dark!