O script simplesmente indica que ele "não teve escolha". Ele tentou configurações mais baixas (o que ela provou ser resistente) e ela estava decidida a alcançar sua vítima:
Riker lowers the phaser, extends his other hand toward Yuta --
Riker : come on... (Yuta pauses) - I'm... sorry.
As Yuta whirls, reaches to touch Chorgan -- and Riker FIRES.
The stun charge shakes Yuta up -- but she remains conscious, recovers her footing. Riker changes the phaser setting, FIRES again. It knocks Yuta back. But she keeps coming.
Riker has no choice. He adjusts the phaser --
RIKER : Don't --
Yuta reaches for Chorgan again -- And Riker FIRES. Setting eight. Vaporize. And Yuta's gone.