Certain Death

• WWizards of the Coast2016-07-13
  Official Ruling
  The targeted creature will have already left the battlefield before its controller loses 2 life and you gain 2 life. If it has any abilities that care about gaining or losing life, they won’t apply.
• WWizards of the Coast2016-07-13
  Official Ruling
  If the creature becomes an illegal target for Certain Death, no player gains or loses life. If it’s a legal target but isn’t destroyed, most likely due to having indestructible, its controller still loses 2 life and you gain 2 life.

Rulings via Scryfall