If you cast a spell on a Mirrorwing Dragon as its only target, that spell will be copied for and target each of your creatures.
Whenever a player casts an instant or sorcery spell that targets only Mirrorwing Dragon, that player copies that spell for each other creature he or she controls that the spell could target.
"Her or she" refers to the caster of the spell. See also the first ruling on the gatherer page for Mirrorwing Dragon:
The copies are only created targeting creatures that the spell’s controller controls. Copies are not created for all creatures on the battlefield, and the affected creatures may be controlled by a different player than the controller of Mirrorwing Dragon. Notably, if you cast Murder targeting your opponent’s Mirrorwing Dragon, your creatures will each get a Murder, not your opponent’s.
Alternatively, since you said that "he killed one of my creatures*", you may be confusing the creature sacrifice cost of Bone Splinters with its effect. After the sacrifice, however, the spell would still be copied for each of your creatures.