Oracle Text

Until end of turn, creatures you control gain "When this creature dies, it deals 2 damage to target creature an opponent controls."

Card Rulings

4/15/2013 Creatures that come under your control after Showstopper resolves won’t have the ability.
4/15/2013 If another player gains control of a creature you control after Showstopper resolves, and then later that turn that creature dies, the creature’s controller when it died controls the triggered ability. That player chooses a creature that one of their opponents controls as a target.