Riker shoots her three times. The first time, she goes stumbling back.
The second time, she almost collapses in pain (and would have if she hadn't grabbed the table). And that was after a quick burst. A sustained burst would have Yuta on the floor in agony, unable to hurt Chorgan.
But no, despite the fact Riker has a means of sending Yuta reeling, he chooses to kill her instead.
I see what you are saying, but what if that second shot was a "kill" setting? (Phasers were so much simpler when there was just stun and evaporate, they didn't leave corpses, now they just do enough damage to kill but there are bodies.) So, back on track, that first shot didn't stun her and then she's hit on a stronger setting that effectively cooked her liver and she wouldn't have survived much longer, but long enough to kill by touching. Then vaporizing her is just finishing her off quicker and preventing her corpse from killing, because even if she was dead, that would still have killed Chorgan. Just for discussion sake, I don't know this, of course.