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.
You don't need to go that far forward, though. It took Christians <400 years to promulgate the Edict of Thessalonica that made Christianity (and of a very particular kind at that!) to be the only legal religion. And one can argue that it's no coincidence that it happened pretty much as soon as they have gained the political upper hand in the Roman Empire.
> Same for Russians and Mongols, there's a pretty large gap with a ton of events in between
Not really. Muscovy was still paying tribute to the Golden Horde and recognizing their supreme authority under Ivan III. His grandson Ivan IV ("the Terrible") conquered the Tatar state, making its lands such as Kazan part of his empire, and sent an expedition to start the conquest of Siberia.
The inaccuracies here are not so much with timing, more so with lack of precision wrt the groups involved. In general, though, I think it's fair to say that, for most part of human history, the oppressed become the oppressors pretty much as soon as they are capable of it.