Lets put the atrocities to one side for a moment. We should also consider the fact that in many cases colonial invasions put into place highly extractive political institutions and extractive economic institutions. That's relevant for the long term. In almost all cases, the colonial governments put into place winner-take-all environments. That meant, if you were willing to be the corrupt puppet native leader, and were willing to supress your other natives at all costs, then you got to be "King". The structures created in those colonial countries still maintain such winner-take-all society which results in authoritarian/dictatorial leaders.