If I use Star Crossed to return an enemy unit that is being targeted to receive the might from Deathgrip, does the unit that was killed to buff still die?

To answer your question: Yes, the friendly unit targeted by Deathgrip still dies, but the second friendly unit does not receive the Might bonus.

The Sequence of Events

1. Playing Deathgrip: You play Deathgrip targeting a friendly unit to kill and another friendly unit to receive a Might bonus. Deathgrip is placed on the chain. Both units are locked as targets when the spell is placed on the chain (FAQ #8401, #9315).
2. Playing Star-Crossed: In response, your opponent plays Star-Crossed, targeting the friendly unit you chose to receive the Might bonus.
3. Resolution of Star-Crossed: The chain resolves LIFO (Last-In, First-Out). Star-Crossed resolves first, and the friendly unit targeted to receive the Might bonus is returned to your hand.
4. Resolution of Deathgrip: Deathgrip now attempts to resolve.
   • Partial Resolution: Because only one of the spell's targets has become illegal (the unit returned to your hand), the spell continues to resolve on all remaining legal targets (Rule 359.3.e.8). The unit targeted to be killed remains a legal target on the board and is destroyed.
   • Might Bonus: Because the "If you do" condition for the Might bonus is dependent on the successful destruction of the target unit (FAQ #8401, #8668), and the unit intended to receive the bonus is no longer on the board, the Might bonus is not applied.
   • Draw: You still draw one card, as the draw instruction is not contingent on the successful execution of the effects (FAQ #8668).

Summary

Because the unit targeted to receive the Might bonus is moved to a non-board zone before the spell resolves, the target becomes invalid for that specific effect. However, because the unit targeted to be killed remains a valid target, the spell resolves partially, killing the unit, but failing to apply the Might bonus.

Note: I could not fully verify this ruling against the retrieved rules.