Yup. But she should have been expecting a heavy guard around the two "unconscious patients". So, would she have been trying to shoot her way through? Crude and not necessarily very effective - she'd have to come up with one hell of a cover story when other guards rushed in and saw her standing on top a heap of corpses, or in an empty but charred room where previously there had been multiple guards and two important patients. If she did have such a plan, why didn't she act on it? Why didn't she vaporize Kirk and Spock on the spot?
One might argue that she did have a plan that would allow her to come out clean after killing the guards and patients. But when, instead of surprising a bunch of nameless guards, she herself was surprised by Kirk and Spock, she must have realized that the secret was out. In that sense, it's perfectly logical that she did not shoot Spock; she'd have won nothing by that, as her cover was blown and her survival extremely unlikely even if she killed the top officers.
One is then only left to wonder what her cunning original plan was. Come in and kill everybody with her phaser, then blame somebody else? Come in, stun some people, subtly kill some others, and then blame somebody else? Come in, subtly kill the patients with a hypospray or somesuch while the guards notice nothing, and only pack the phaser as an insurance if something goes wrong?
She might also have been attempting a suicide mission: she would be exposed and killed, but the specifics of the conspiracy would be safe. But that's unlikely on two levels: one, the two assassin goons wouldn't have known anything beyond those facts that would incriminate Valeris, and two, she didn't opt for a suicidal rampage once she was exposed.
Timo Saloniemi