If my opponent plays Star-Crossed and in response I kill his unit before it resolves, do I still have to return my unit?

Yes, you still have to return your unit.

Even though you killed the opponent's unit, Star-Crossed does not "fizzle" (fail entirely). It resolves as much as possible, ignoring only the instruction or portion of the instruction that targeted the now-invalid entity.

Sequence of Events

1. Opponent plays Star-Crossed: They target your unit and their unit. Star-Crossed is added to the chain.
2. You play an effect to kill their unit: You kill the opponent's unit. This effect is added to the top of the chain.
3. Chain State (Top to Bottom):
   • [Top] Your kill effect.
   • [Bottom] Star-Crossed.
4. Resolution (LIFO - Last-In, First-Out):
   • Your kill effect resolves first: The opponent's unit is moved to the trash.
   • Star-Crossed resolves: It checks for its targets. Because the opponent's unit is no longer on the board, that part of the instruction cannot be followed and is ignored. However, your unit remains a legal target, so the spell proceeds to return your unit to your hand.

This follows Rule 359.3.e.8, which states that if an instruction has more than one target and fewer than all of them become unavailable, the instruction will still execute on the remaining valid targets. This is also consistent with RiftJudge FAQ #10058 and #10311, which specifically address this interaction.