Marlow says that the heads only show Kurtz's lack of restraint. Is it true? At the point, is Marlow a reliable character?

Marlow becomes a reliable character the moment he doubts his romantic obsession with Kurtz as a counterculture mystic. Marlow sees Kurtz for the greedy man that he is. Killing other natives is just one part of Kurtz's lack of restraint. Kurtz had set himself up as a king or demi-god. He had allowed himself to be swallowed up by the darkness and relent to the madness of the jungle.