I think the key focus you should take is the differences between the context of colonial Africa and then Vietnam during the Vietnam war. What is fascinating about the film is how relevant the context is to the same kind of ideas expressed in the book - Africa and Vietnam are both presented as arenas outside of the control of civilisation where the foreigners there - be they colonialists or American soldiers - are free to do whatever they want. Thus we have absolutely mad scenes of a ship firing shells into the heart of Africa senselessly and likewise the battle where American soldiers are surfing in the sea whilst being fired upon.
However, I guess the focus of Copolla is much more the weariness of war and its impact on the American soldier - the desire to go home, the friends and family and loved ones left back home - that drive the action of the film. It could be said that in the senseless killing that we observe all that is emphasised is the tremendous fragility of the youth of the American soldiers, who should never have left home, and are put in a position way beyond their maturity. It is the kind of pressures that they have to cope with through the setting that breaks them and leads to the kind of insane shots we are presented with.