To help me narrow down exactly what you're looking for, could you specify:
You learn Avada Kedavra from Sebastian Sallow during his relationship questline, specifically when visiting the catacombs, which is a late-game event. avadakedavra.part12.rar
Is this a (e.g., from FanFiction.net or Wattpad)? Is this a mod file for a game (e.g., Hogwarts Legacy)? What platform (site/app) did you see this file name on? To help me narrow down exactly what you're
If you turn Sebastian in to Ominis, it may prevent you from learning the spell unless you talk to him before he is imprisoned. What platform (site/app) did you see this file name on
There are stories like Avada Kedavra and Rise Again on fanfiction platforms, which detail dark themes and alternative plotlines.
To learn the spell, you should choose "Everyone should know that spell" when prompted by Sebastian. If you miss it, you can often talk to him in the Undercroft later.