Re: Does it hurt to be shot by phaser?
General musings: Valeris supposedly was highly positioned in the conspiracy, as she knew key facts no random hitman need know. She'd thus probably have a lot riding on it. But then comes the "Logically, you have to fire" scene - and she doesn't kill Spock.
Three ways to read that. One, she was decent to the point of illogic, and would rather see millions suffer in the distance (the thing the conspiracy supposedly wanted to avoid, by shifting all the suffering to the Klingons instead) than inflict damage on somebody she can see, hear and touch. Two, she always was in the conspiracy for reasons of pure personal benefit (what those might be, we don't know), and sacrificing herself to the cause would not be part of her agenda under any circumstances. Three, she saw immediately that Spock was wrong/lying, and that killing him and Kirk and the rest would not benefit the conspiracy at this point.
She just couldn't bring herself to kill her mentor, even though it would have been the "logical" thing to do. She was too emotionally attached to Spock, despite being a Vulcan. She respected and admired (and I daresay loved) him too much. Remember, the character was originally supposed to be Saavik.
Watch out, or I'll get you with my Andorian ice powers.