Why didn’t Riker do something else in The Vengeance Factor?

Question

In the final scene of Star Trek: The Next Generation, "The Vengeance Factor" (episode 9 of season 3), why did Riker

have to kill Yuta?

It is clearly said in previous scenes (by Dr. Beverly Crusher) that a microvirus, embodied in Yuta, was only harmful to members of the clan with which Yuta’s clan was fighting. If so, couldn't he simply use force? Like catch her, and block her from reaching Brull or doing anything less harmful? If he wouldn't have enough force, I think that Worf was somewhere close to help him.

In my opinion, "The Vengeance Factor" is one of the best episodes of Star Trek: The Next Generation. But its final scene is simply terrible. Was it really necessary to end it in such a poor / lame / American-western-like way (where every "evil" must eventually die at the movie’s end)?

Answer 1

The script simply indicates that he had "no choice". He'd tried lower settings (which she'd proven resistant to) and she was intent on reaching her victim:

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.

Answer 2

The script (in Valorum's answer) explains that Riker had no choice, and perhaps if the scene was set up better that would come across better. (Very weird that everyone else stays seated during an attempted murder.)

But having "no choice" does not explain why Yuta cannot be stopped by the phaser on stun. One explanation could be that her species is naturally more immune to being stunned. The script says:

Riker MATERIALIZES, phaser in hand. Everyone's astounded. The Gatherer guard goes for his phaser rifle. Riker FIRES, stunning the guard. ... Riker aims the phaser at Chorgan, who freezes, glares at Riker. The guard is already starting to shake off the effects of the stun.

The script also provides the villain a moment to reveal her unnatural physical state:

"I was the one chosen... transformed... my cells were altered... my aging slowed... enough to finish my task."

So, although it is not definitively explained, it's possible that her natural biology combined with altered cells made her immune to being stunned, forcing Riker to go up to setting eight.