I think Picard just sitting there while she gets vaporized was due to a special effect which required it, but yeah that scene is strange. You'd think the phaser would have a setting that knocks you severely unconscious rather than instant vaporization. Or, you know, just keep firing stun shots until she stops getting up.
Also think to myself, "why didn't he just tackle her after stunning?" Maybe the poison was so deadly he couldn't risk it but it still strikes me as an odd moment.
This episode reminds me a lot of "Legacy", but that one is more memorable to me because of Riker's conversation with Data about betrayal.