Christians were persecuted by the Roman Empire, then became conquerors of the world.
Russians were oppressed by the Mongols, then became conquerors of Eurasia.
Communists were oppressed by Tsarists, then became ruthless oppressors themselves.
Protestants were oppressed in Europe, so they set sail to America and became oppressors of the natives.
The better explanation is simple and banal - power concentration makes people abuse it.
My list of examples is very similar to this one and the ven diagram here is "was oppressed became oppressor"... in most cases it appears that only if the oppressed are destroyed or I would argue in the case of America- controlled at the margins... then they don't circle back around to abuse their newly acquired power.