It just randomly occured to me... Isn't a Colony Ship technically an embarked pop?
So... isn't it technically an atrocity comparable to purging a pop when someone makes it go boom in space?
Colony Ships are not taking pops from planets, so you can not use them to Purge unwanted pops using them.
The initial pop of a colony is kind of appearing from "nowhere". Even with Cryogenics, keeping 500 million to 1 billion humans on a colonyship would be hard.
I asume the bulk of the initial pop is being ferried in via civilian transports (the same ones doing Migration and Resettlement work).
Heck, one of the original trailers has a human Colonyship being killed due to being asumed hostile. So nope, not anywhere near that kind of atrocity.
At tops it would be on the level of Terror Bombing for one pop.