Why would murder equate sadism? Even in the loosest interpretation of the word, sadism involves taking pleasure in an action - but conducting an action and taking pleasure in an action are fundamentally unrelated things.
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.
As for the phaser angle of all this... Valeris knows it's an ambush already, down at the sickbay, so firing a kill phaser at Spock and tripping the alarms should not bother her much. But perhaps it's an ambush set up by a very small cabal of trusted officers (since the very fact of setting up the ambush reveals that Kirk and his friends don't know for sure who their enemies are)? In that case, Valeris could kill them all and make it look as if some other assassin X dunnit; she might benefit from not tripping the alarms in that case. Plenty of ways to kill a victim in Sickbay if Valeris stuns him first with a phaser....