It is hard to know for sure why some countries became independent through peaceful means while others had to fight.  I would argue that it was largely the attitude of the colonizer that determined whether the decolonization would be peaceful.  However, the nature of the independence movement and the geographical location of the colony mattered in some instances.

It seems clear from history that some colonial powers were much more willing than others to give up their colonies.  Perhaps the best example of a power that did not want to give its colonies up willingly was France.  France fought very nasty wars in both Vietnam and Algeria.  There was also violence in such places as Madagascar and Cameroon.  By contrast, there was much less violence in the decolonization of the British colonies.  India was allowed to go with little violence.  So were most of England’s African colonies.  The difference seems to stem mainly from the attitudes of the colonial powers.  We can speculate that England, which was not defeated in WWII, felt less of a need to maintain an empire to prove itself as a major power.  By contrast, France’s defeat in WWII might have led it to hold on to its colonies more strongly.

There were some other factors that came into play.  For example, the US was willing to grant independence to the Philippines, but helped France in its struggle to hold on to Vietnam.  This was partly because the Vietnamese independence movement appeared to be communist-dominated while the Filipinos who would be likely to lead the country after independence were anti-communist.  Vietnam’s geographic location may have mattered here too, since it neighbors communist China where the Philippines does not.

Overall, though, I would argue that the degree of violence had more to do with the attitudes of the colonizers than with anything else.

Did the nature of decolonization depend more on the native populations or the colonial powers?

Although both the attitudes of the colonial powers and the attitudes of the natives mattered, I would say that the colonial powers were more important in determining whether decolonization would be peaceful in any given colony.  My main piece of evidence for this is the fact that some colonial powers had many violent decolonizations while other powers did had few or none.

My main examples of colonial powers are France and England.  These are the best examples since they each had many colonies and because their experiences with decolonization were very different.  France had colonies in Asia and Africa.  In both areas, there were examples of colonial wars.  The most violent of these wars were the wars in Vietnam and in Algeria.  By contrast, Britain did not have even one decolonization experience that was as violent as either of these wars. 

It is possible that France simply had bad luck with violent natives.  However, this seems unlikely.  It is unlikely that any native group would have preferred to fight if it could get what it wanted without fighting.  The difference, in my view, is that Britain was willing to give in peacefully to people like Gandhi and Nehru while France was not willing to give in peacefully to people like Ho Chi Minh.  The French felt that they needed Vietnam for political and economic reasons.  They felt that they could not give Algeria up because they felt it was an integral part of France.  By contrast, Britain did not feel that it had to hold India or countries like Nigeria at all costs.

In my view, the major factor that determined whether decolonization was violent was the attitude of the colonial power towards the particular colony.