Although the themes in Heart of Darkness, by Conrad, are universal (that is, there is no doubt about what Conrad is discussing), there is no one correct interpretation of his text. Thus, the two movies, Heart of Darkness and Apocalypse Now.
From reading the text, it is evident that Conrad is discussing the topics of: purity and goodness are abstract ideas that can never really exist and the spreading of society breeds corruption.
Purity, goodness, and righteousness are abstract ideas that do not exist in reality; this is what Conrad is clearly eluding to in Heart of Darkness. This is evident by the constant comparison of 'Whiteness' and 'Darkness'. Although 'whiteness' and 'darkness' are on opposite ends of the spectrum, Conrad often interchanges the two; implying they are one in the same (or at least a mixture of the two abstract ideas). Good...Bad? The answer to this question depends on where you fit into society; your role in society dictates your outlook in life.
By trying to achieve purity and righteousness, groups of individuals (the elite) are often blinded by their foolishness. More often than not, the act of spreading a civilized society (or culture) subjects other groups to cruel and inhumane acts by the righteous group (although, this was not their {the elite's} intention). Since language is abstract and no two people see things the same, purity, true goodness, and righteousness can never be obtained. Thus, spreading and defining the rules of a righteous society is a contradiction, because some group will always be at a disadvantage.
In the two movies, both topics are evident. The apparent 'whiteness' is a mask for the 'evilness' of the two empires (British and US). Both empires thrive on the oppression of groups. In Heart of Darkness, Kurtz (and people like Marlow) spread the corrupt ideals of the British Empire by exploiting the natural resources of Africa. In Apocalypse Now, the US spreads corruption by the eradication of weaker groups through its war efforts.