Not sure if I would lump all those up together, these examples are overly broad and have little in common. There's more than a thousand years and basically no causal link between Roman persecution of early Christians and Crusades, let alone European imperialism, especially if you take Ethiopian, Greek, Georgian, and Armenian Christians into account. Same for Russians and Mongols, there's a pretty large gap with a ton of events in between, and Mongol Empire was humongous to begin with, it wasn't about Rus' in particular. And communists that became ruthless oppressors were already radicalized during the persecution, it was literally the radical wing of a militant faction of a huge umbrella party that included people that would have felt right at home in modern EU (e.g. Kollontai and her early activism).
The better explanation is simple and banal - power concentration makes people abuse it.
> There's more than a thousand years and basically no causal link between Roman persecution of early Christians and Crusades
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.
I think that a lot of monotheistic religions, including Christianity, are generally intolerant to other branches and religions, especially when the faith is supposed to represent absolute truth, so it's probably unrelated to the history of persecution. And Muscovy wasn't the only land opressed by Mongols.
>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.
Doesn't this also hold for non-oppressed that have the opportunity? Although I suppose it'd be hard to find any examples of non-oppressed groups. Pillaging or conquering neighbors was pretty much the norm throughout the history. Rus' was converted to Christianity in part to stop raids such as Siege of Constantinople of 860.
I wouldn't consider this "lumping the groups together", or that they must exist together in time... its likely a group may require many generations before they can "oppress" another group.
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.
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.