If you cast Macabre Mockery during a player’s end step, the creature will remain on the battlefield until the next turn’s end step. However, the +2/+0 and haste wear off during the cleanup step (which comes after the end step) of the turn in which Macabre Mockery resolved.

In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, the creature you control from Macabre Mockery’s effect is exiled.