Heart of Darkness is in its entirety not an allegory. Its surface is too profound and meaningful to allow itself to be interpreted in more than two ways. There are however several parts in the novel that hint at the opposite and that prove that the context of the novel can be seen from more than one angle.
This can mainly be perceived in the life of Mr. Kurtz, as his descent into madness can be seen as an allegory for the colonization and destruction of the African continent and its people by the Europeans. Because just as Kurtz was a perfectly sane and normal man before he went into the African wilderness so were also the European nations very civilized before they came to Africa. And just as the nations of Europe governed Africa and its people without rules and restrictions so did isolation compel Mr. Kurtz to live his life without boundaries.
Proof of this can be found on page 83, "His mother was half-English his father was half-French. All of Europe contributed to the making of Kurtz." This shows that Kurtz can be seen as a symbol of Europe, as he is the typical European explorer - ambitious, greedy and adventurous.
And if Kurtz is Europe embodied then it would be quite logical that Africa should be represented by the "harlequin" (p.87) his faithful Russian comrade. Their relation greatly resembles that of the relation between Africa and Europe, although the African people act quite differently to their "masters" compared to how the Russian acted towards Kurtz. The similarities lay in how the master treats the subordinate. "He wanted to shoot me too one day....I had a small lot of ivory...he wanted it, and wouldn't hear reason...and there was nothing on earth to prevent him killing whom...