I'm surprised there's any debate about this at all. Watch the video. After the second shot, she collapses to her knees. After this, Riker increases the power setting again, hitting the button at least three (if not four) times. That means he skipped settings between 'bring a humanoid to his or her knees' and 'vaporize said humanoid.' It's just really bad writing. It's pretty transparent that the writers wanted a certain ending to this episode.
When I last watched this episode a few years back, I thought it was over-the-top and completely unnecessary. It felt forced. I watched the scene on YouTube yesterday to refresh my memory, and it's bad as I remember it (if not worse) - Riker didn't have to kill her. Like several others have said, the second shot showed that he didn't even have to go beyond that power setting.
Let's answer some questions:
1. Would she have gone any further if he kept shooting her on that power setting? Watch the video again. I don't think so.
2. If you really don't think that second power setting was enough, could he not simply have selected a setting that was higher than the second shot but lower than the third? Yes. It probably would have caused her to completely collapse on the floor from pain.
He kills her only because the writers decided it would be more dramatic, and a 'better' ending to the episode.