Tropico 6

Start a sandbox game from Colonial Era (where you have only 2 factions) and go for revolutionary faction and kill opposite faction citizens.

To see the list of people in a faction, press and select "Almanach" (the green book).

On the first topic, select a faction (on the second line), then select people one by one and choose "Kill".

You must be really fast and never stop to choose the option to kill people !

If you have only 50 citizens, 90% is 45/50 so speed is the key !

I want to add my version of how to get the achievement, even though there is already one solution that works. The benefit of my solution is that you do not have to get out of your way to get it, you can do it on any mission that is at least in Cold War era.
Edit: In theory this should work in Cold War era, but I was only able to do it in modern times era for some reason.
1. Get your economy up and runnig, skip this if you play in sandbox with unlimited money
2. Get a Pirate Cove and start a heist for the Brandenburg Gate, place it when it's done.
3. Get a Research Station and research TV-Station work modes in the Cold War era tab and the Free Housing edict in the Colonial era tab. You can also grab Light Bulb Ban in Modern Times era to help with energy consumption.
3. Build a lot of housing which is electrified - it is very important that the housing uses power as the next step will not work if they don't.
4. Get at least one TV-Station and activate a work mode for the one faction you want everyone to join. For me I chose "We Watch" so people join the communist faction, as I had the most approval with that faction. Set the budget for the TV-Station(s) to max to increase efficiency.
5. Make sure your people are happy so they don't emigrate (alternatively you can research Emigration in Cold War era to block emigration in total - but I have not tested it wit this) and get an Immigration Office and set the workmode to Tropico First. This is to stop too much fluctuation between immigration and emigration.
6. The achievement will unlock after a couple of years. I did it in combination with the "We don't have time for that" achievement and this one unlocked slightly after the 20 required years. I did not play with unlimited money though, so it can be a lot faster.

So what is happening here? - The Brandenburg Gate does not allow die-hard political views which means that everyone can be influenced to join another faction. The TV-Station can change political views of people living in electrified buildings, but can not change die-hard political views - which we got rid of with the Brandenburg Gate. The free housing edict makes it that everyone can use housing of any quality, so even broke citizen will live in electrified buildings. After that it's just a matter of time until the TV-Station ifluenced everyone to join the faction of your choice.
For me this worked perfectly and I think this is actually the "intended" way of unlocking the achievement. If it does not work for you please leave a comment and I will see if I can figure out a workaround.

Have a great day :)