If i kill someone whilst i have taken cards from them with my It That betrays, where do their cards go? im unfamiliar with the ruling and i think that this scenario is unique.
Under C.R. 800.4a, if a player leaves the game, all objects owned by them leave the game. Thus, for example, if you bring a card from that player's graveyard to the battlefield with It That Betrays, and that player then leaves the game, that card will leave the game along with that player.
EDIT (Jul. 19): By "killing" I assume you mean "causing a player to lose the game". A player who loses the game leaves that game (C.R. 104.5).
EDIT (Jul. 19): By "killing" I assume you mean "causing a player to lose the game". A player who loses the game leaves that game (C.R. 104.5).