So? The fact that Riker warned her twice did not mean he had to set the phaser to kill. Riker knew that a quick zap would send her reeling back (the second time he shot her). All he had to do was to hit her with a sustained shot, say five seconds, at that same exact power level, and she would have been easily rendered incapacitated or unconscious. All he has to do then is grab her, get a bunch of other folks to hold her so she can't attack Chorgan, then beam her up to the Enterprise and put in the brig. Chorgan and Marouk conclude their negotiations with her in custody, then when they take Marouk home, they turn Yuta over to the Acamarian authorities.
But instead, Riker chose to kill her.