I will award 20 mana for each answer.
This question is inspired by the following excerpt from "I, Asimov: A Memoir" by Isaac Asimov, and all of the examples listed are excluded as answers.
"Right now there is an influx of Soviet Jews into Israel. They are fleeing because they expect religious persecution. Yet at the instant their feet touched Israeli soil, they became extreme Israeli nationalists with no pity for the Palestinians. From persecuted to persecutors in the blinking of an eye. The Jews are not remarkable for this. It’s just that because I’m a Jew I am sensitive to this particular situation—but it is a general phenomenon. When pagan Rome persecuted the early Christians, the Christians pleaded for tolerance. When Christianity took over, was there tolerance? Not on your life. The persecution began at once in the other direction. The Bulgarians demanded freedom for themselves from an oppressive regime and made use of that freedom by attacking the ethnic Turks in their midst. The Azerbaijani demanded freedom from the centralized control of the Soviet Union, but they seemed to want to make use of that freedom to kill all the Armenians in their midst. The Bible says that those who have experienced persecution should not in their turn persecute: “Thou shalt neither vex a stranger, nor oppress him; for ye were strangers in the land of Egypt” (Exodus 22:21). Yet who follows that text?"
1,000Freed slaves sent to Liberia immediately started enslaving the native Africans.
A few decades after liberation from Imperial Japanese occupation, the South Korean government was rounding up homeless and disabled people into concentration camps to make the streets more presentable for foreign visitors.
In medieval/renaissance England, pretty much every time the monarch switched between Protestant and Catholic, they started persecuting the other group.
Yugoslavia in the 1990s. Ethnic groups in Yugoslavia, particularly Croats and Bosniaks, faced historical periods of discrimination and repression, especially during World War II. During the Yugoslav Wars in the 1990s, these groups, now in positions of power within their respective republics, engaged in ethnic cleansing and persecution against other ethnicities, notably during the Bosnian War.
It's somewhat related to an example Azimov already gave in your excerpt, but in the 1990s the Armenians in Nagorno-Karabach ethnically cleansed all Azerbaijanis from that region to form the Republic of Artsakh, and just this past year almost the entire Armenian population has been expelled from Nagorno-Karabach by the Azerbaijanis after invading and dissolving Artsakh.
Cambodia under the Khmer Rouge. The Khmer Rouge, originally a marginalized political group, faced suppression under the Cambodian government and American bombing during the Vietnam War. Upon seizing power in 1975, the Khmer Rouge, led by Pol Pot, implemented radical policies leading to the genocide of nearly 2 million people, targeting ethnic minorities, intellectuals, and perceived political enemies.
The Pilgrims. They came from Brownist and other Congregationalist sects who resisted outlawing and the increasingly Catholic practices imposed by the Church of England, but within 16 years of establishing Plymouth Colony, they launched a punitive expedition against the natives of Block Island beginning the Pequot War and the expropriation of the natives of New England.
Iraqi Shiites were horribly oppressed under the Hussein regime. Now they've taken over the country and may not have much sympathy for their former ruling class, nor for the Kurdish minority
Cuban Americans, they fled the horrible Castro regime in tiny boats, yet on arrival turned to the Republican party, now threatening the greatest immigrant round up and expulsion of all time in 2024, and have been Republican and anti-immigrant for decades